Data Engineer Interview Questions and Answers

All interview questions
#	Question	Topic	Difficulty	Type	Practise
1	How do you handle task failures and alerting in Airflow?	Airflow	Medium	scenario, conceptual	—
2	How do you make one Airflow DAG depend on another?	Airflow	Medium	architecture, conceptual	—
3	How do you test Airflow DAGs before they reach production?	Airflow	Medium	coding, conceptual	—
4	How should Airflow retries and idempotency work together?	Airflow	Medium	conceptual, scenario	—
5	How would you process a variable number of files each day in Airflow?	Airflow	Medium	coding, scenario	—
6	Poke, reschedule or deferrable: how should an Airflow sensor wait?	Airflow	Medium	conceptual, optimization	—
7	What are the logical date and data interval in Airflow, and what changed in Airflow 3?	Airflow	Medium	conceptual, debugging	—
8	What is the difference between catchup and backfill in Airflow 3?	Airflow	Medium	conceptual, scenario	—
9	When should Airflow tasks pass data through XCom, and when through external storage?	Airflow	Medium	conceptual, scenario	—
10	Airflow is slow and tasks sit in scheduled or queued. How do you find the bottleneck?	Airflow	Hard	debugging, optimization	—
11	How do you choose an Airflow executor: Local, Celery, Kubernetes or several?	Airflow	Hard	architecture, scenario	—
12	What changed in Airflow 3, and how would you migrate Airflow 2 DAGs?	Airflow	Hard	conceptual, scenario	—
13	How would you design the S3 layout and partitioning for a data lake?	AWS	Medium	architecture, scenario	—
14	ETL vs ELT: what factors decide the choice?	Data Engineering	Easy	conceptual, architecture	—
15	Batch or streaming: how do you choose for a use case?	Data Engineering	Medium	conceptual, architecture	—
16	Data lake vs warehouse vs lakehouse: when would you choose each?	Data Engineering	Medium	conceptual, architecture	—
17	How do you backfill a pipeline safely?	Data Engineering	Medium	scenario, architecture	—
18	How do you handle late-arriving data?	Data Engineering	Medium	scenario, architecture	—
19	How do you handle PII in a data pipeline?	Data Engineering	Medium	scenario, architecture	—
20	How do you handle schema changes from upstream sources in a pipeline?	Data Engineering	Medium	scenario, architecture	—
21	How do you test a data pipeline?	Data Engineering	Medium	conceptual, scenario	—
22	How would you choose a partitioning strategy for a large table?	Data Engineering	Medium	architecture, optimization	—
23	How would you choose an orchestrator for data pipelines?	Data Engineering	Medium	conceptual, architecture	—
24	How would you design an idempotent batch pipeline?	Data Engineering	Medium	architecture, scenario	—
25	How would you design an incremental load for a large table?	Data Engineering	Medium	architecture, scenario	—
26	How would you investigate a suddenly slower data pipeline?	Data Engineering	Medium	debugging, scenario	—
27	How would you monitor pipeline freshness and data SLAs?	Data Engineering	Medium	scenario, architecture	—
28	What are the most important data-quality checks in production?	Data Engineering	Medium	conceptual, scenario	—
29	What does CI/CD look like for data pipelines?	Data Engineering	Medium	conceptual, architecture	—
30	Exactly-once vs effectively-once: what can a pipeline really guarantee?	Data Engineering	Hard	conceptual, architecture	—
31	How would you design a CDC pipeline?	Data Engineering	Hard	architecture, scenario	—
32	What is Unity Catalog used for?	Databricks	Easy	conceptual	—
33	What is Delta Lake time travel good for, and what are its limits?	Delta Lake	Easy	conceptual, scenario	—
34	What problems does Delta Lake solve?	Delta Lake	Easy	conceptual	—
35	Delta Lake vs Apache Iceberg vs Apache Hudi: how do they differ?	Delta Lake	Medium	conceptual, architecture	—
36	How does Delta Lake MERGE work, and why do concurrent MERGEs fail?	Delta Lake	Medium	conceptual, debugging	—
37	How is a Parquet file organised, and how does that speed up queries?	Delta Lake	Medium	conceptual, optimization	—
38	OPTIMIZE, Z-ORDER or liquid clustering: which would you use on a Delta table?	Delta Lake	Medium	conceptual, optimization, scenario	—
39	What does VACUUM do in Delta Lake, and what can go wrong?	Delta Lake	Medium	conceptual, scenario	—
40	What is schema evolution and when is it safe?	Delta Lake	Medium	conceptual, scenario	—
41	Backspace String Compare: Read Both Strings Backwards With Skip Counters	DSA	Easy	coding	LeetCode (opens in a new tab)
42	Balanced Binary Tree: Height Check with Early Exit	DSA	Easy	coding	LeetCode (opens in a new tab)
43	Best Time to Buy and Sell Stock: Maximum Profit From One Trade	DSA	Easy	coding	LeetCode (opens in a new tab)
44	Binary Search: Find a Target in a Sorted Array in O(log n)	DSA	Easy	coding	LeetCode (opens in a new tab)
45	Ceil in Sorted Array: Lower-Bound Binary Search	DSA	Easy	coding	GeeksforGeeks (opens in a new tab)
46	Climbing Stairs: Count Ways with a Fibonacci-Style DP	DSA	Easy	coding	LeetCode (opens in a new tab)
47	Complement of Base 10 Integer: Flip Bits With an All-Ones Mask	DSA	Easy	coding	LeetCode (opens in a new tab)
48	Contains Duplicate: Detect Whether Any Value Appears Twice	DSA	Easy	coding	LeetCode (opens in a new tab)
49	Convert Sorted Array to BST: Pick the Middle, Recurse on Halves	DSA	Easy	coding	LeetCode (opens in a new tab)
50	Counting Bits: Set-Bit Counts for 0 to n with a One-Line DP	DSA	Easy	coding	LeetCode (opens in a new tab)
51	Diameter of Binary Tree: Longest Path via Post-Order Heights	DSA	Easy	coding	LeetCode (opens in a new tab)
52	Find All Numbers Disappeared in an Array: Cyclic Sort and In-Place Marking	DSA	Easy	coding	LeetCode (opens in a new tab)
53	Find if Path Exists in Graph: Reachability with BFS or Union-Find	DSA	Easy	coding	LeetCode (opens in a new tab)
54	Find Pivot Index: Balance Left and Right Prefix Sums	DSA	Easy	coding	LeetCode (opens in a new tab)
55	Find Rotation Count: The Index of the Minimum Is the Answer	DSA	Easy	coding	GeeksforGeeks (opens in a new tab)
56	First Unique Character in a String: Count, Then Scan Again	DSA	Easy	coding	LeetCode (opens in a new tab)
57	Flood Fill: Repaint a Connected Region with DFS or BFS	DSA	Easy	coding	LeetCode (opens in a new tab)
58	Happy Number: Detect a Cycle in a Digit-Square Sequence	DSA	Easy	coding	LeetCode (opens in a new tab)
59	Implement Queue using Stacks: FIFO With Two Stacks in Amortised O(1)	DSA	Easy	coding	LeetCode (opens in a new tab)
60	Implement Stack using Queues: LIFO Behaviour From FIFO Parts	DSA	Easy	coding	LeetCode (opens in a new tab)
61	Invert Binary Tree: Recursive and Iterative Mirror Solutions	DSA	Easy	coding	LeetCode (opens in a new tab)
62	Island Perimeter: Count Land Edges Without a Traversal	DSA	Easy	coding	LeetCode (opens in a new tab)
63	Kth Largest Element in a Stream: Size-k Min-Heap	DSA	Easy	coding	LeetCode (opens in a new tab)
64	Last Stone Weight: Simulating Smashes with a Max-Heap	DSA	Easy	coding	LeetCode (opens in a new tab)
65	Linked List Cycle: Floyd's Fast and Slow Pointers	DSA	Easy	coding	LeetCode (opens in a new tab)
66	Longest Common Prefix: Shared Start of a List of Strings	DSA	Easy	coding	LeetCode (opens in a new tab)
67	Longest Palindrome: Pair Up Letter Counts and Keep One Centre	DSA	Easy	coding	LeetCode (opens in a new tab)
68	Majority Element: Find the Value That Fills More Than Half the Array	DSA	Easy	coding	LeetCode (opens in a new tab)
69	Max Sum Subarray of Size K: The Fixed-Size Sliding Window	DSA	Easy	coding	GeeksforGeeks (opens in a new tab)
70	Maximum Depth of Binary Tree: Recursive DFS, Iterative DFS and BFS	DSA	Easy	coding	LeetCode (opens in a new tab)
71	Meeting Rooms: Check Whether One Person Can Attend Every Meeting	DSA	Easy	coding	GeeksforGeeks (opens in a new tab)
72	Merge Two Sorted Lists: Dummy Head Iteration and Recursion	DSA	Easy	coding	LeetCode (opens in a new tab)
73	Middle of the Linked List: Find It in One Pass With Fast and Slow Pointers	DSA	Easy	coding	LeetCode (opens in a new tab)
74	Min Cost Climbing Stairs: Cheapest Path to the Top with 1D DP	DSA	Easy	coding	LeetCode (opens in a new tab)
75	Missing Number: Find the One Value Absent From 0 to n	DSA	Easy	coding	LeetCode (opens in a new tab)
76	Move Zeroes: Shift Zeros to the End While Keeping Order	DSA	Easy	coding	LeetCode (opens in a new tab)
77	Next Greater Element I: First Larger Value to the Right	DSA	Easy	coding	LeetCode (opens in a new tab)
78	Number of 1 Bits: Count Set Bits in an Integer	DSA	Easy	coding	LeetCode (opens in a new tab)
79	Number of Occurrence: Count a Value with Two Boundary Searches	DSA	Easy	coding	GeeksforGeeks (opens in a new tab)
80	Number of Recent Calls: Count Requests in a Sliding Time Window	DSA	Easy	coding	LeetCode (opens in a new tab)
81	Palindrome Linked List: Find the Middle, Reverse the Back Half, Compare	DSA	Easy	coding	LeetCode (opens in a new tab)
82	Ransom Note: Check Letter Supply Against Demand With a Counter	DSA	Easy	coding	LeetCode (opens in a new tab)
83	Remove All Adjacent Duplicates In String: Cancel Pairs With a Stack	DSA	Easy	coding	LeetCode (opens in a new tab)
84	Remove Duplicates from Sorted Array: Compact Unique Values In Place	DSA	Easy	coding	LeetCode (opens in a new tab)
85	Reverse Bits: Mirror the 32 Bits of an Unsigned Integer	DSA	Easy	coding	LeetCode (opens in a new tab)
86	Reverse Linked List: Iterative and Recursive Solutions	DSA	Easy	coding	LeetCode (opens in a new tab)
87	Reverse String: Reverse a Character Array in Place	DSA	Easy	coding	LeetCode (opens in a new tab)
88	Same Tree: Comparing Two Binary Trees Recursively and Iteratively	DSA	Easy	coding	LeetCode (opens in a new tab)
89	Segregate 0s and 1s: Partition a Binary Array in One Pass	DSA	Easy	coding	GeeksforGeeks (opens in a new tab)
90	Set Mismatch: Find the Duplicate and the Missing Number With Cyclic Sort	DSA	Easy	coding	LeetCode (opens in a new tab)
91	Single Number: Find the Value That Appears Only Once Using XOR	DSA	Easy	coding	LeetCode (opens in a new tab)
92	Squares of a Sorted Array: Merge From Both Ends Without Sorting	DSA	Easy	coding	LeetCode (opens in a new tab)
93	Subtree of Another Tree: Brute-Force Matching and Linear Serialisation	DSA	Easy	coding	LeetCode (opens in a new tab)
94	Two Sum: Find Two Indices That Add Up to a Target	DSA	Easy	coding	LeetCode (opens in a new tab)
95	Valid Anagram: Check Whether Two Strings Use the Same Letters	DSA	Easy	coding	LeetCode (opens in a new tab)
96	Valid Palindrome II: Palindrome After Deleting at Most One Character	DSA	Easy	coding	LeetCode (opens in a new tab)
97	Valid Palindrome: Check a Phrase While Ignoring Case and Punctuation	DSA	Easy	coding	LeetCode (opens in a new tab)
98	Valid Parentheses: Check That Every Bracket Closes in the Right Order	DSA	Easy	coding	LeetCode (opens in a new tab)
99	0 - 1 Knapsack Problem: Take or Skip Each Item Once	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
100	132 Pattern: Find a Low, High, Middle Subsequence in One Pass	DSA	Medium	coding	LeetCode (opens in a new tab)
101	3Sum Closest: Sort, Fix One Value, and Squeeze Two Pointers	DSA	Medium	coding	LeetCode (opens in a new tab)
102	3Sum: Find All Unique Triplets That Sum to Zero	DSA	Medium	coding	LeetCode (opens in a new tab)
103	4Sum: Two Fixed Indices, Two Pointers and Careful Duplicate Skipping	DSA	Medium	coding	LeetCode (opens in a new tab)
104	Add Two Numbers: Digit-by-Digit Addition on Linked Lists	DSA	Medium	coding	LeetCode (opens in a new tab)
105	Aggressive Cows: Maximise the Minimum Gap with Binary Search	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
106	Best Time to Buy and Sell Stock with Cooldown: State-Machine DP	DSA	Medium	coding	LeetCode (opens in a new tab)
107	Binary Tree Level Order Traversal: BFS with a Queue, Level by Level	DSA	Medium	coding	LeetCode (opens in a new tab)
108	Binary Tree Right Side View: Last Node of Each Level	DSA	Medium	coding	LeetCode (opens in a new tab)
109	Capacity To Ship Packages Within D Days: Binary Search on Capacity	DSA	Medium	coding	LeetCode (opens in a new tab)
110	Car Fleet: Count the Groups of Cars Arriving at a Destination	DSA	Medium	coding	LeetCode (opens in a new tab)
111	Cheapest Flights Within K Stops: Bounded Bellman-Ford for Limited Hops	DSA	Medium	coding	LeetCode (opens in a new tab)
112	Circular Array Loop: Fast and Slow Pointers on Index Jumps	DSA	Medium	coding	LeetCode (opens in a new tab)
113	Clone Graph: Deep-Copy a Connected Graph with a Hash Map and BFS or DFS	DSA	Medium	coding	LeetCode (opens in a new tab)
114	Coin Change II: Count Combinations That Make an Amount	DSA	Medium	coding	LeetCode (opens in a new tab)
115	Coin Change: Fewest Coins with Bottom-Up DP	DSA	Medium	coding	LeetCode (opens in a new tab)
116	Combination Sum II: Use Each Value Once and Avoid Duplicate Answers	DSA	Medium	coding	LeetCode (opens in a new tab)
117	Combination Sum: Reach a Target with Reusable Values via Backtracking	DSA	Medium	coding	LeetCode (opens in a new tab)
118	Construct Binary Tree from Preorder and Inorder Traversal	DSA	Medium	coding	LeetCode (opens in a new tab)
119	Container With Most Water: Maximise Area Between Two Lines	DSA	Medium	coding	LeetCode (opens in a new tab)
120	Contiguous Array: Longest Balanced 0/1 Subarray via Prefix Sums	DSA	Medium	coding	LeetCode (opens in a new tab)
121	Copy List with Random Pointer: Hash Map and Interleaving Solutions	DSA	Medium	coding	LeetCode (opens in a new tab)
122	Count Good Nodes in Binary Tree: DFS Carrying the Path Maximum	DSA	Medium	coding	LeetCode (opens in a new tab)
123	Count Subsets with Sum: Add Up the Ways, Not Just True or False	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
124	Course Schedule II: Return a Valid Course Order with Topological Sort	DSA	Medium	coding	LeetCode (opens in a new tab)
125	Course Schedule: Detect a Cycle in a Prerequisite Graph with Kahn's Algorithm	DSA	Medium	coding	LeetCode (opens in a new tab)
126	Daily Temperatures: Days Until a Warmer Day With a Monotonic Stack	DSA	Medium	coding	LeetCode (opens in a new tab)
127	Decode Ways: Count Digit-to-Letter Decodings with Prefix DP	DSA	Medium	coding	LeetCode (opens in a new tab)
128	Delete Node in a BST: Leaf, One Child and Two Children Cases	DSA	Medium	coding	LeetCode (opens in a new tab)
129	Design Add and Search Words: Trie Search with Wildcards	DSA	Medium	coding, architecture	LeetCode (opens in a new tab)
130	Design Circular Queue: A Fixed-Capacity Ring Buffer	DSA	Medium	coding	LeetCode (opens in a new tab)
131	Design Twitter: News Feed with a K-Way Heap Merge	DSA	Medium	coding, architecture	LeetCode (opens in a new tab)
132	Edit Distance: Levenshtein Distance with a 2D DP Table	DSA	Medium	coding	LeetCode (opens in a new tab)
133	Encode and Decode Strings: Serialise a List of Strings Safely	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
134	Evaluate Reverse Polish Notation: Compute a Postfix Expression	DSA	Medium	coding	LeetCode (opens in a new tab)
135	Extra Characters in a String: Dynamic Programming over a Trie of Words	DSA	Medium	coding	LeetCode (opens in a new tab)
136	Find All Anagrams in a String: Fixed Window With Letter Counts	DSA	Medium	coding	LeetCode (opens in a new tab)
137	Find All Duplicates in an Array: Cyclic Sort and Sign Marking in O(1) Space	DSA	Medium	coding	LeetCode (opens in a new tab)
138	Find First and Last Position of a Value: Lower and Upper Bound Binary Search	DSA	Medium	coding	LeetCode (opens in a new tab)
139	Find K Closest Elements: Binary Search for the Best Window	DSA	Medium	coding	LeetCode (opens in a new tab)
140	Find Minimum in Rotated Sorted Array: Binary Search Against the Right End	DSA	Medium	coding	LeetCode (opens in a new tab)
141	Find Peak Element: Follow the Uphill Side with Binary Search	DSA	Medium	coding	LeetCode (opens in a new tab)
142	Find the Duplicate Number: Cycle Detection on an Array	DSA	Medium	coding	LeetCode (opens in a new tab)
143	Fruit Into Baskets: Longest Window With at Most Two Distinct Values	DSA	Medium	coding	LeetCode (opens in a new tab)
144	Gas Station: Find the Start of a Circular Route in One Greedy Pass	DSA	Medium	coding	LeetCode (opens in a new tab)
145	Generate Parentheses: List Every Balanced String of n Pairs	DSA	Medium	coding	LeetCode (opens in a new tab)
146	Graph Valid Tree: Check Edge Count, Connectivity and Cycles with Union-Find	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
147	Group Anagrams: Bucket Words That Share the Same Letters	DSA	Medium	coding	LeetCode (opens in a new tab)
148	Hand of Straights: Group Cards into Consecutive Runs Greedily	DSA	Medium	coding	LeetCode (opens in a new tab)
149	House Robber II: Non-Adjacent Maximum Sum When Houses Form a Circle	DSA	Medium	coding	LeetCode (opens in a new tab)
150	House Robber: Maximum Non-Adjacent Sum with Take-or-Skip DP	DSA	Medium	coding	LeetCode (opens in a new tab)
151	Implement Trie (Prefix Tree): Insert, Search and Prefix Lookup	DSA	Medium	coding, architecture	LeetCode (opens in a new tab)
152	Insert Interval: Add a Range to a Sorted List and Merge Overlaps	DSA	Medium	coding	LeetCode (opens in a new tab)
153	Insert into a BST: Walk Down to the Empty Spot	DSA	Medium	coding	LeetCode (opens in a new tab)
154	Interleaving String: Can Two Strings Merge into a Third? 2D DP	DSA	Medium	coding	LeetCode (opens in a new tab)
155	Interval List Intersections: Two Pointers over Sorted Ranges	DSA	Medium	coding	LeetCode (opens in a new tab)
156	Is Graph Bipartite?: Two-Colour Every Component	DSA	Medium	coding	LeetCode (opens in a new tab)
157	Jump Game II: Fewest Jumps to the End with Greedy Level-by-Level BFS	DSA	Medium	coding	LeetCode (opens in a new tab)
158	Jump Game: Can You Reach the Last Index? Greedy Furthest Reach	DSA	Medium	coding	LeetCode (opens in a new tab)
159	K Closest Points to Origin: Bounded Max-Heap and Quickselect	DSA	Medium	coding	LeetCode (opens in a new tab)
160	Koko Eating Bananas: Binary Search on the Answer	DSA	Medium	coding	LeetCode (opens in a new tab)
161	Kth Largest Element in an Array: Heap, Quickselect and Counting	DSA	Medium	coding	LeetCode (opens in a new tab)
162	Kth Smallest Element in a BST: Inorder Traversal with Early Stop	DSA	Medium	coding	LeetCode (opens in a new tab)
163	Kth Smallest Element in a Sorted Matrix: Binary Search on Values	DSA	Medium	coding	LeetCode (opens in a new tab)
164	Letter Case Permutation: Branch Twice on Every Letter	DSA	Medium	coding	LeetCode (opens in a new tab)
165	Letter Combinations of a Phone Number: Cartesian Product by Backtracking	DSA	Medium	coding	LeetCode (opens in a new tab)
166	Linked List Cycle II: Find Where the Cycle Starts With Floyd's Algorithm	DSA	Medium	coding	LeetCode (opens in a new tab)
167	Longest Common Subsequence: The Classic Two-String DP Table	DSA	Medium	coding	LeetCode (opens in a new tab)
168	Longest Consecutive Sequence: Longest Run of Consecutive Integers in O(n)	DSA	Medium	coding	LeetCode (opens in a new tab)
169	Longest Increasing Subsequence: O(n^2) DP and O(n log n) Patience Sorting	DSA	Medium	coding	LeetCode (opens in a new tab)
170	Longest K Unique Characters Substring: A Window Bounded by a Distinct Count	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
171	Longest Palindromic Substring: Expand Around Centres or Interval DP	DSA	Medium	coding	LeetCode (opens in a new tab)
172	Longest Repeating Character Replacement: Window With At Most k Changes	DSA	Medium	coding	LeetCode (opens in a new tab)
173	Longest Substring Without Repeating Characters: Sliding Window	DSA	Medium	coding	LeetCode (opens in a new tab)
174	Lowest Common Ancestor of a Binary Tree: One Post-Order Search	DSA	Medium	coding	LeetCode (opens in a new tab)
175	Lowest Common Ancestor of a BST: Follow the Split Point	DSA	Medium	coding	LeetCode (opens in a new tab)
176	LRU Cache: Hash Map Plus Doubly Linked List for O(1) Operations	DSA	Medium	coding, architecture	LeetCode (opens in a new tab)
177	Max Area of Island: Largest Connected Land Region with Flood Fill	DSA	Medium	coding	LeetCode (opens in a new tab)
178	Max Consecutive Ones III: Longest Window With at Most K Zeros	DSA	Medium	coding	LeetCode (opens in a new tab)
179	Maximum Absolute Sum of Any Subarray: Run Kadane for Max and Min	DSA	Medium	coding	LeetCode (opens in a new tab)
180	Maximum Candies Allocated to K Children: Binary Search on the Share	DSA	Medium	coding	LeetCode (opens in a new tab)
181	Maximum Length of Pair Chain: Greedy by Earliest End	DSA	Medium	coding	LeetCode (opens in a new tab)
182	Maximum Product Subarray: Track Both the Largest and Smallest Product	DSA	Medium	coding	LeetCode (opens in a new tab)
183	Maximum Subarray Sum with One Deletion: Kadane With Two States	DSA	Medium	coding	LeetCode (opens in a new tab)
184	Maximum Subarray: Largest Sum of a Contiguous Slice with Kadane's Algorithm	DSA	Medium	coding	LeetCode (opens in a new tab)
185	Maximum Sum Circular Subarray: Kadane Plus Total Minus Minimum	DSA	Medium	coding	LeetCode (opens in a new tab)
186	Maximum Sum Combination: The K Largest Pair Sums with a Max-Heap	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
187	Meeting Rooms II: Count the Rooms with a Min-Heap or a Sweep Line	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
188	Merge Intervals: Combine Every Overlapping Range	DSA	Medium	coding	LeetCode (opens in a new tab)
189	Merge Triplets to Form Target: Keep Only Triplets That Never Overshoot	DSA	Medium	coding	LeetCode (opens in a new tab)
190	Min Cost to Connect All Points: Minimum Spanning Tree with Prim or Kruskal	DSA	Medium	coding	LeetCode (opens in a new tab)
191	Min Cost to Connect Ropes: Always Join the Two Shortest	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
192	Min Stack: A Stack That Returns Its Minimum in Constant Time	DSA	Medium	coding	LeetCode (opens in a new tab)
193	Minimum Add to Make Parentheses Valid: Count Unmatched Brackets in One Pass	DSA	Medium	coding	LeetCode (opens in a new tab)
194	Minimum Height Trees: Trim Leaves Until the Centre Remains	DSA	Medium	coding	LeetCode (opens in a new tab)
195	Minimum Number of Days to Make m Bouquets: Binary Search on the Day	DSA	Medium	coding	LeetCode (opens in a new tab)
196	Minimum Size Subarray Sum: Grow Until Valid, Then Shrink	DSA	Medium	coding	LeetCode (opens in a new tab)
197	My Calendar I: Accept Bookings Only When They Do Not Overlap	DSA	Medium	coding	LeetCode (opens in a new tab)
198	Network Delay Time: Single-Source Shortest Paths with Dijkstra's Algorithm	DSA	Medium	coding	LeetCode (opens in a new tab)
199	Next Greater Element II: Monotonic Stack Over a Circular Array	DSA	Medium	coding	LeetCode (opens in a new tab)
200	Non-overlapping Intervals: Fewest Removals to Eliminate Overlaps	DSA	Medium	coding	LeetCode (opens in a new tab)
201	Number of Closed Islands: Sink the Border, Then Count	DSA	Medium	coding	LeetCode (opens in a new tab)
202	Number of Connected Components in an Undirected Graph with Union-Find	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
203	Number of Islands: Count Connected Land Cells with BFS, DFS or Union-Find	DSA	Medium	coding	LeetCode (opens in a new tab)
204	Number of Provinces: Count Components in an Adjacency Matrix	DSA	Medium	coding	LeetCode (opens in a new tab)
205	Pacific Atlantic Water Flow: Reverse Multi-Source BFS from Both Oceans	DSA	Medium	coding	LeetCode (opens in a new tab)
206	Palindrome Partitioning: Split a String into Palindromes by Backtracking	DSA	Medium	coding	LeetCode (opens in a new tab)
207	Palindromic Substrings: Count Every Palindrome by Expanding Around Centres	DSA	Medium	coding	LeetCode (opens in a new tab)
208	Partition Equal Subset Sum: 0/1 Knapsack on Half the Total	DSA	Medium	coding	LeetCode (opens in a new tab)
209	Partition Labels: Split a String So Each Letter Stays in One Part	DSA	Medium	coding	LeetCode (opens in a new tab)
210	Path With Minimum Effort: Minimise the Largest Step on a Grid	DSA	Medium	coding	LeetCode (opens in a new tab)
211	Peak Index in a Mountain Array: Binary Search on the Slope	DSA	Medium	coding	LeetCode (opens in a new tab)
212	Permutation in String: Does One String Contain a Rearrangement of Another?	DSA	Medium	coding	LeetCode (opens in a new tab)
213	Permutations: Every Ordering of Distinct Values with Backtracking	DSA	Medium	coding	LeetCode (opens in a new tab)
214	Product of Array Except Self: Prefix and Suffix Products Without Division	DSA	Medium	coding	LeetCode (opens in a new tab)
215	Redundant Connection: Find the Edge That Closes a Cycle with Union-Find	DSA	Medium	coding	LeetCode (opens in a new tab)
216	Remove All Adjacent Duplicates in String II: Stack of Character Counts	DSA	Medium	coding	LeetCode (opens in a new tab)
217	Remove Duplicate Letters: Smallest Order with a Greedy Monotonic Stack	DSA	Medium	coding	LeetCode (opens in a new tab)
218	Remove K Digits: Smallest Number With a Greedy Monotonic Stack	DSA	Medium	coding	LeetCode (opens in a new tab)
219	Remove Nodes From Linked List: Keep Nodes With Nothing Larger to Their Right	DSA	Medium	coding	LeetCode (opens in a new tab)
220	Remove Nth Node From End of List: One Pass with a Gap of n	DSA	Medium	coding	LeetCode (opens in a new tab)
221	Reorder List: Find the Middle, Reverse, Then Interleave	DSA	Medium	coding	LeetCode (opens in a new tab)
222	Reorganize String: Place the Most Frequent Character First, Never Twice in a Row	DSA	Medium	coding	LeetCode (opens in a new tab)
223	Reverse Linked List II: Reverse a Sublist in One Pass With a Dummy Node	DSA	Medium	coding	LeetCode (opens in a new tab)
224	Rotate Image: Turn a Square Matrix 90 Degrees Clockwise in Place	DSA	Medium	coding	LeetCode (opens in a new tab)
225	Rotate List: Close the Ring, Then Cut It at the New Tail	DSA	Medium	coding	LeetCode (opens in a new tab)
226	Rotting Oranges: Minutes to Spread via Level-by-Level Multi-Source BFS	DSA	Medium	coding	LeetCode (opens in a new tab)
227	Search a 2D Matrix II: Staircase Search from the Top-Right Corner	DSA	Medium	coding	LeetCode (opens in a new tab)
228	Search a 2D Matrix: Binary Search over a Flattened Sorted Grid	DSA	Medium	coding	LeetCode (opens in a new tab)
229	Search in Rotated Sorted Array: One-Pass Binary Search	DSA	Medium	coding	LeetCode (opens in a new tab)
230	Search Suggestions System: Autocomplete with a Trie or Sorted Binary Search	DSA	Medium	coding	LeetCode (opens in a new tab)
231	Set Matrix Zeroes: Zero Out Rows and Columns in Place	DSA	Medium	coding	LeetCode (opens in a new tab)
232	Shortest Unsorted Continuous Subarray: Find the Boundaries With Running Max and Min	DSA	Medium	coding	LeetCode (opens in a new tab)
233	Simplify Path: Canonical Unix Paths With a Stack of Directory Names	DSA	Medium	coding	LeetCode (opens in a new tab)
234	Single Number III: Split Two Unique Values With One XOR Bit	DSA	Medium	coding	LeetCode (opens in a new tab)
235	Sort Characters By Frequency: Count, Then Order by a Heap or Buckets	DSA	Medium	coding	LeetCode (opens in a new tab)
236	Sort Colors: One-Pass Three-Way Partition of 0s, 1s and 2s	DSA	Medium	coding	LeetCode (opens in a new tab)
237	Spiral Matrix: Read a Grid in Clockwise Spiral Order	DSA	Medium	coding	LeetCode (opens in a new tab)
238	String to Integer (atoi): Parse a Signed 32-bit Integer by Hand	DSA	Medium	coding	LeetCode (opens in a new tab)
239	Subarray Product Less Than K: Count Windows Ending at Each Index	DSA	Medium	coding	LeetCode (opens in a new tab)
240	Subarray Sum Equals K: Count Contiguous Slices With a Given Sum	DSA	Medium	coding	LeetCode (opens in a new tab)
241	Subarray Sums Divisible by K: Count Matching Prefix Remainders	DSA	Medium	coding	LeetCode (opens in a new tab)
242	Subset Sum Problem: Track Every Reachable Total	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
243	Subsets II: Power Set Without Duplicates When Values Repeat	DSA	Medium	coding	LeetCode (opens in a new tab)
244	Subsets: Generate the Power Set with Include/Exclude Backtracking	DSA	Medium	coding	LeetCode (opens in a new tab)
245	Sum of Two Integers: Add Without the Plus or Minus Operator	DSA	Medium	coding	LeetCode (opens in a new tab)
246	Surrounded Regions: Capture Enclosed Cells by Flood-Filling from the Border	DSA	Medium	coding	LeetCode (opens in a new tab)
247	Swap Nodes in Pairs: Rewire Every Two Nodes With a Dummy Head	DSA	Medium	coding	LeetCode (opens in a new tab)
248	Target Sum: Count Sign Assignments with Subset-Sum DP	DSA	Medium	coding	LeetCode (opens in a new tab)
249	Task Scheduler: Cooldowns with a Max-Heap or a Counting Formula	DSA	Medium	coding	LeetCode (opens in a new tab)
250	Time Based Key-Value Store: Versioned Lookups with Binary Search	DSA	Medium	coding, architecture	LeetCode (opens in a new tab)
251	Top K Frequent Elements: Return the k Most Common Values	DSA	Medium	coding	LeetCode (opens in a new tab)
252	Topological Sort: Order a DAG with Kahn's Algorithm or DFS	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
253	Triplets with Smaller Sum: Count Many Pairs at Once With Two Pointers	DSA	Medium	coding	GeeksforGeeks (opens in a new tab)
254	Two Sum II: Find a Pair With a Target Sum in a Sorted Array	DSA	Medium	coding	LeetCode (opens in a new tab)
255	Unique Paths: Count Grid Routes with 2D DP or a Binomial Coefficient	DSA	Medium	coding	LeetCode (opens in a new tab)
256	Valid Parenthesis String: Track a Range of Open Counts Greedily	DSA	Medium	coding	LeetCode (opens in a new tab)
257	Valid Sudoku: Check Rows, Columns and Boxes for Repeated Digits	DSA	Medium	coding	LeetCode (opens in a new tab)
258	Validate Binary Search Tree: Bounds Recursion and Inorder Check	DSA	Medium	coding	LeetCode (opens in a new tab)
259	Word Break: Can a String Be Split into Dictionary Words? Prefix DP	DSA	Medium	coding	LeetCode (opens in a new tab)
260	Word Search: Trace a Word Through a Letter Grid with DFS Backtracking	DSA	Medium	coding	LeetCode (opens in a new tab)
261	Alien Dictionary: Derive a Letter Order from Sorted Words with Topological Sort	DSA	Hard	coding	GeeksforGeeks (opens in a new tab)
262	Binary Tree Maximum Path Sum: Post-Order Gains with a Global Best	DSA	Hard	coding	LeetCode (opens in a new tab)
263	Burst Balloons: Interval DP by Choosing the Last Balloon to Burst	DSA	Hard	coding	LeetCode (opens in a new tab)
264	Find Median from Data Stream: Two Heaps Kept in Balance	DSA	Hard	coding	LeetCode (opens in a new tab)
265	First Missing Positive: Cyclic Sort Into Home Slots in O(n) Time, O(1) Space	DSA	Hard	coding	LeetCode (opens in a new tab)
266	IPO: Maximise Capital by Picking the Best Affordable Project Each Round	DSA	Hard	coding	LeetCode (opens in a new tab)
267	Largest Rectangle in Histogram: Biggest Area Under the Bars	DSA	Hard	coding	LeetCode (opens in a new tab)
268	Maximum Frequency Stack: Pop the Most Frequent Value in O(1)	DSA	Hard	coding, architecture	LeetCode (opens in a new tab)
269	Median of Two Sorted Arrays: Partition Binary Search in O(log min(m, n))	DSA	Hard	coding	LeetCode (opens in a new tab)
270	Merge k Sorted Lists: Min-Heap and Divide-and-Conquer Solutions	DSA	Hard	coding	LeetCode (opens in a new tab)
271	Minimum Window Substring: Shortest Slice Containing Every Required Character	DSA	Hard	coding	LeetCode (opens in a new tab)
272	N-Queens: Place Non-Attacking Queens Row by Row with Backtracking	DSA	Hard	coding	LeetCode (opens in a new tab)
273	Partition Into 2 Subsets with Min Sum Diff: Subset Sum Up to Half the Total	DSA	Hard	coding	GeeksforGeeks (opens in a new tab)
274	Reconstruct Itinerary: Eulerian Path with Hierholzer's Algorithm	DSA	Hard	coding	LeetCode (opens in a new tab)
275	Regular Expression Matching: Dot and Star with a 2D DP Table	DSA	Hard	coding	LeetCode (opens in a new tab)
276	Reverse Nodes in k-Group: In-Place Group Reversal on a Linked List	DSA	Hard	coding	LeetCode (opens in a new tab)
277	Serialize and Deserialize Binary Tree: Preorder with Null Markers	DSA	Hard	coding	LeetCode (opens in a new tab)
278	Shortest Subarray with Sum at Least K: Prefix Sums and a Monotonic Deque	DSA	Hard	coding	LeetCode (opens in a new tab)
279	Sliding Window Maximum: Max of Every Window With a Monotonic Deque	DSA	Hard	coding	LeetCode (opens in a new tab)
280	Sliding Window Median: Two Heaps with Lazy Deletion	DSA	Hard	coding	LeetCode (opens in a new tab)
281	Smallest Range Covering Elements from K Lists: K-Way Merge with a Min-Heap	DSA	Hard	coding	LeetCode (opens in a new tab)
282	Split Array Largest Sum: Binary Search on the Largest Part	DSA	Hard	coding	LeetCode (opens in a new tab)
283	Substring with Concatenation of All Words: Word-Sized Sliding Windows per Offset	DSA	Hard	coding	LeetCode (opens in a new tab)
284	Sudoku Solver: Backtracking with Constraint Sets and Smart Cell Choice	DSA	Hard	coding	LeetCode (opens in a new tab)
285	Swim in Rising Water: Minimax Path with a Modified Dijkstra	DSA	Hard	coding	LeetCode (opens in a new tab)
286	Trapping Rain Water: Total Water Held Between Elevation Bars	DSA	Hard	coding	LeetCode (opens in a new tab)
287	Word Ladder: Shortest Word Transformation with BFS and Wildcard Buckets	DSA	Hard	coding	LeetCode (opens in a new tab)
288	Word Search II: Find Many Words in a Grid with a Trie and Backtracking	DSA	Hard	coding	LeetCode (opens in a new tab)
289	Consumer lag keeps growing. How do you diagnose and fix it?	Kafka	Medium	debugging, scenario	—
290	Explain Kafka partitions and consumer groups.	Kafka	Medium	conceptual, architecture	—
291	How do acks, replication factor and min.insync.replicas decide Kafka durability?	Kafka	Medium	conceptual, scenario	—
292	How do you choose retention and size disk for a Kafka topic?	Kafka	Medium	architecture, scenario	—
293	How do you choose the number of partitions for a Kafka topic?	Kafka	Medium	architecture, scenario	—
294	How do you evolve Kafka message schemas safely with a Schema Registry?	Kafka	Medium	conceptual, scenario	—
295	How do you handle poison messages and dead-letter topics in Kafka?	Kafka	Medium	scenario, debugging	—
296	How does Kafka guarantee message ordering, and where can ordering break?	Kafka	Medium	conceptual, scenario	—
297	Kafka vs Kinesis vs a message queue: how do you choose?	Kafka	Medium	architecture, conceptual	—
298	What causes consumer group rebalances, and how do you reduce their impact?	Kafka	Medium	debugging, conceptual	—
299	What is at-least-once delivery and what problems can it create?	Kafka	Medium	conceptual, scenario	—
300	What is Kafka log compaction, and when would you use it?	Kafka	Medium	conceptual, scenario	—
301	How would you stream database changes into Kafka with Debezium?	Kafka	Hard	architecture, scenario	—
302	What does exactly-once mean in Kafka, and how is it achieved?	Kafka	Hard	conceptual, architecture	—
303	When should you avoid Python UDFs in PySpark?	PySpark	Medium	conceptual, optimization	—
304	When would you use a broadcast join in Spark?	PySpark	Medium	conceptual, optimization	—
305	Explain shallow copy vs deep copy.	Python	Easy	conceptual, debugging	—
306	List vs tuple vs set: when would you use each in a data pipeline?	Python	Easy	conceptual	—
307	What is a generator and why can it help with large datasets?	Python	Easy	conceptual, coding	—
308	Why are mutable default arguments dangerous in Python?	Python	Easy	conceptual, debugging	—
309	How should exceptions be handled in production data pipelines?	Python	Medium	conceptual, scenario	—
310	How would you add retries with backoff to API ingestion in Python?	Python	Medium	coding, scenario	—
311	How would you process a 50 GB CSV file on a machine with 8 GB of RAM?	Python	Medium	scenario, coding	—
312	When would you not use pandas in a data pipeline?	Python	Medium	conceptual, scenario	—
313	Why are dict and set lookups O(1) in Python, and when are they not?	Python	Medium	conceptual, optimization	—
314	What are Snowflake virtual warehouses?	Snowflake	Easy	conceptual	—
315	What caches does Snowflake use, and when does each one help?	Snowflake	Easy	conceptual, scenario	—
316	What is the difference between Snowflake Time Travel and Fail-safe?	Snowflake	Easy	conceptual, scenario	—
317	What is zero-copy cloning in Snowflake, and how would you use it?	Snowflake	Easy	conceptual, scenario	—
318	How do micro-partitions affect Snowflake query performance?	Snowflake	Medium	conceptual, optimization	—
319	How do you build an incremental pipeline with Snowflake streams and tasks?	Snowflake	Medium	architecture, coding	—
320	Should you scale a Snowflake warehouse up or out?	Snowflake	Medium	optimization, scenario	—
321	When should you define a clustering key on a Snowflake table?	Snowflake	Medium	optimization, scenario	—
322	What is the difference between a transformation and an action in Spark?	Apache Spark	Easy	conceptual	—
323	Explain Spark jobs, stages and tasks.	Apache Spark	Medium	conceptual	—
324	How does partition count affect Spark performance?	Apache Spark	Medium	optimization, conceptual	—
325	What causes a shuffle in Spark?	Apache Spark	Medium	conceptual	—
326	What is data skew and how can you mitigate it?	Apache Spark	Hard	optimization, debugging	—
327	Explain INNER JOIN vs LEFT JOIN with a practical example.	SQL	Easy	conceptual, coding	—
328	Find customers who never placed an order, and explain NOT IN vs NOT EXISTS.	SQL	Easy	coding, debugging	LeetCode (opens in a new tab)
329	How do window functions differ from GROUP BY?	SQL	Easy	conceptual, coding	—
330	Average Order Value: SQL Case Study with 8 Approaches	SQL	Medium	coding, optimization, scenario	—
331	Customer Lifetime Value: SQL Case Study with 8 Approaches	SQL	Medium	coding, optimization, scenario	—
332	Daily Active Users: SQL Case Study with 8 Approaches	SQL	Medium	coding, optimization, scenario	—
333	Find the second-highest salary without using a simple MAX approach.	SQL	Medium	coding	LeetCode (opens in a new tab)
334	How would you detect and remove duplicate records safely?	SQL	Medium	coding, scenario	LeetCode (opens in a new tab)
335	Lead Scoring: SQL Case Study with 7 Approaches	SQL	Medium	coding, scenario, optimization	—
336	New vs Returning Customers: SQL Case Study with 8 Approaches	SQL	Medium	coding, optimization, scenario	—
337	Refund Analysis: SQL Case Study with 7 Approaches	SQL	Medium	coding, optimization, scenario	—
338	Return the top three salaries in each department, including ties.	SQL	Medium	coding	LeetCode (opens in a new tab)
339	Revenue doubled after you joined a second table. What happened and how do you fix it?	SQL	Medium	debugging, coding	—
340	Session Duration: SQL Case Study with 8 Approaches	SQL	Medium	coding, optimization, scenario	—
341	Subscription Renewals: SQL Case Study with 7 Approaches	SQL	Medium	coding, optimization, scenario	—
342	Top Selling Products: SQL Case Study with 8 Approaches	SQL	Medium	coding, optimization, scenario	—
343	A/B Test Results: SQL Case Study with 7 Approaches	SQL	Hard	coding, scenario, optimization	—
344	Churn Rate: SQL Case Study with 8 Approaches	SQL	Hard	coding, optimization, scenario	—
345	Conversion Funnel: SQL Case Study with 8 Approaches	SQL	Hard	coding, optimization, scenario	—
346	Employee Hierarchy: SQL Case Study with 8 Approaches	SQL	Hard	coding, optimization, scenario	—
347	Fraud Pattern Detection: SQL Case Study with 8 Approaches	SQL	Hard	coding, optimization, scenario	—
348	How would you optimize a slow analytical SQL query?	SQL	Hard	optimization, scenario	—
349	Inventory Turnover: SQL Case Study with 8 Approaches	SQL	Hard	coding, optimization, scenario	—
350	Monthly Revenue: SQL Case Study with 8 Approaches	SQL	Hard	coding, optimization, scenario	—
351	Write an SCD Type 2 upsert in SQL that closes old versions and inserts new ones.	SQL	Hard	coding, scenario	—

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