DSA Interview Questions for Data Engineers

All DSA questions

DSA interview questions
#	Question	Topic	Difficulty	Type	Practise
1	Majority Element: Find the Value That Fills More Than Half the Array	Arrays	Easy	coding	LeetCode (opens in a new tab)
2	Find Pivot Index: Balance Left and Right Prefix Sums	Arrays	Easy	coding	LeetCode (opens in a new tab)
3	Product of Array Except Self: Prefix and Suffix Products Without Division	Arrays	Medium	coding	LeetCode (opens in a new tab)
4	Subarray Sum Equals K: Count Contiguous Slices With a Given Sum	Arrays	Medium	coding	LeetCode (opens in a new tab)
5	Subarray Sums Divisible by K: Count Matching Prefix Remainders	Arrays	Medium	coding	LeetCode (opens in a new tab)
6	Contiguous Array: Longest Balanced 0/1 Subarray via Prefix Sums	Arrays	Medium	coding	LeetCode (opens in a new tab)
7	Shortest Subarray with Sum at Least K: Prefix Sums and a Monotonic Deque	Arrays	Hard	coding	LeetCode (opens in a new tab)
8	Maximum Subarray: Largest Sum of a Contiguous Slice with Kadane's Algorithm	Arrays	Medium	coding	LeetCode (opens in a new tab)
9	Maximum Product Subarray: Track Both the Largest and Smallest Product	Arrays	Medium	coding	LeetCode (opens in a new tab)
10	Maximum Subarray Sum with One Deletion: Kadane With Two States	Arrays	Medium	coding	LeetCode (opens in a new tab)
11	Maximum Absolute Sum of Any Subarray: Run Kadane for Max and Min	Arrays	Medium	coding	LeetCode (opens in a new tab)
12	Maximum Sum Circular Subarray: Kadane Plus Total Minus Minimum	Arrays	Medium	coding	LeetCode (opens in a new tab)
13	Rotate Image: Turn a Square Matrix 90 Degrees Clockwise in Place	Arrays	Medium	coding	LeetCode (opens in a new tab)
14	Spiral Matrix: Read a Grid in Clockwise Spiral Order	Arrays	Medium	coding	LeetCode (opens in a new tab)
15	Set Matrix Zeroes: Zero Out Rows and Columns in Place	Arrays	Medium	coding	LeetCode (opens in a new tab)
16	Missing Number: Find the One Value Absent From 0 to n	Arrays	Easy	coding	LeetCode (opens in a new tab)
17	Find All Numbers Disappeared in an Array: Cyclic Sort and In-Place Marking	Arrays	Easy	coding	LeetCode (opens in a new tab)
18	Set Mismatch: Find the Duplicate and the Missing Number With Cyclic Sort	Arrays	Easy	coding	LeetCode (opens in a new tab)
19	Find All Duplicates in an Array: Cyclic Sort and Sign Marking in O(1) Space	Arrays	Medium	coding	LeetCode (opens in a new tab)
20	First Missing Positive: Cyclic Sort Into Home Slots in O(n) Time, O(1) Space	Arrays	Hard	coding	LeetCode (opens in a new tab)
21	Contains Duplicate: Detect Whether Any Value Appears Twice	Hashing	Easy	coding	LeetCode (opens in a new tab)
22	Valid Anagram: Check Whether Two Strings Use the Same Letters	Hashing	Easy	coding	LeetCode (opens in a new tab)
23	Two Sum: Find Two Indices That Add Up to a Target	Hashing	Easy	coding	LeetCode (opens in a new tab)
24	First Unique Character in a String: Count, Then Scan Again	Hashing	Easy	coding	LeetCode (opens in a new tab)
25	Longest Palindrome: Pair Up Letter Counts and Keep One Centre	Hashing	Easy	coding	LeetCode (opens in a new tab)
26	Ransom Note: Check Letter Supply Against Demand With a Counter	Hashing	Easy	coding	LeetCode (opens in a new tab)
27	Valid Sudoku: Check Rows, Columns and Boxes for Repeated Digits	Hashing	Medium	coding	LeetCode (opens in a new tab)
28	Group Anagrams: Bucket Words That Share the Same Letters	Hashing	Medium	coding	LeetCode (opens in a new tab)
29	Longest Consecutive Sequence: Longest Run of Consecutive Integers in O(n)	Hashing	Medium	coding	LeetCode (opens in a new tab)
30	Longest Common Prefix: Shared Start of a List of Strings	Strings	Easy	coding	LeetCode (opens in a new tab)
31	Reverse String: Reverse a Character Array in Place	Strings	Easy	coding	LeetCode (opens in a new tab)
32	Encode and Decode Strings: Serialise a List of Strings Safely	Strings	Medium	coding	GeeksforGeeks (opens in a new tab)
33	String to Integer (atoi): Parse a Signed 32-bit Integer by Hand	Strings	Medium	coding	LeetCode (opens in a new tab)
34	Number of 1 Bits: Count Set Bits in an Integer	Bit Manipulation	Easy	coding	LeetCode (opens in a new tab)
35	Reverse Bits: Mirror the 32 Bits of an Unsigned Integer	Bit Manipulation	Easy	coding	LeetCode (opens in a new tab)
36	Single Number: Find the Value That Appears Only Once Using XOR	Bit Manipulation	Easy	coding	LeetCode (opens in a new tab)
37	Counting Bits: Set-Bit Counts for 0 to n with a One-Line DP	Bit Manipulation	Easy	coding	LeetCode (opens in a new tab)
38	Complement of Base 10 Integer: Flip Bits With an All-Ones Mask	Bit Manipulation	Easy	coding	LeetCode (opens in a new tab)
39	Sum of Two Integers: Add Without the Plus or Minus Operator	Bit Manipulation	Medium	coding	LeetCode (opens in a new tab)
40	Single Number III: Split Two Unique Values With One XOR Bit	Bit Manipulation	Medium	coding	LeetCode (opens in a new tab)
41	Valid Palindrome II: Palindrome After Deleting at Most One Character	Two Pointers	Easy	coding	LeetCode (opens in a new tab)
42	Valid Palindrome: Check a Phrase While Ignoring Case and Punctuation	Two Pointers	Easy	coding	LeetCode (opens in a new tab)
43	Move Zeroes: Shift Zeros to the End While Keeping Order	Two Pointers	Easy	coding	LeetCode (opens in a new tab)
44	Remove Duplicates from Sorted Array: Compact Unique Values In Place	Two Pointers	Easy	coding	LeetCode (opens in a new tab)
45	Squares of a Sorted Array: Merge From Both Ends Without Sorting	Two Pointers	Easy	coding	LeetCode (opens in a new tab)
46	Segregate 0s and 1s: Partition a Binary Array in One Pass	Two Pointers	Easy	coding	GeeksforGeeks (opens in a new tab)
47	Backspace String Compare: Read Both Strings Backwards With Skip Counters	Two Pointers	Easy	coding	LeetCode (opens in a new tab)
48	Two Sum II: Find a Pair With a Target Sum in a Sorted Array	Two Pointers	Medium	coding	LeetCode (opens in a new tab)
49	3Sum: Find All Unique Triplets That Sum to Zero	Two Pointers	Medium	coding	LeetCode (opens in a new tab)
50	Container With Most Water: Maximise Area Between Two Lines	Two Pointers	Medium	coding	LeetCode (opens in a new tab)
51	Sort Colors: One-Pass Three-Way Partition of 0s, 1s and 2s	Two Pointers	Medium	coding	LeetCode (opens in a new tab)
52	3Sum Closest: Sort, Fix One Value, and Squeeze Two Pointers	Two Pointers	Medium	coding	LeetCode (opens in a new tab)
53	Triplets with Smaller Sum: Count Many Pairs at Once With Two Pointers	Two Pointers	Medium	coding	GeeksforGeeks (opens in a new tab)
54	Subarray Product Less Than K: Count Windows Ending at Each Index	Two Pointers	Medium	coding	LeetCode (opens in a new tab)
55	4Sum: Two Fixed Indices, Two Pointers and Careful Duplicate Skipping	Two Pointers	Medium	coding	LeetCode (opens in a new tab)
56	Shortest Unsorted Continuous Subarray: Find the Boundaries With Running Max and Min	Two Pointers	Medium	coding	LeetCode (opens in a new tab)
57	Trapping Rain Water: Total Water Held Between Elevation Bars	Two Pointers	Hard	coding	LeetCode (opens in a new tab)
58	Implement Stack using Queues: LIFO Behaviour From FIFO Parts	Queue	Easy	coding	LeetCode (opens in a new tab)
59	Implement Queue using Stacks: FIFO With Two Stacks in Amortised O(1)	Queue	Easy	coding	LeetCode (opens in a new tab)
60	Number of Recent Calls: Count Requests in a Sliding Time Window	Queue	Easy	coding	LeetCode (opens in a new tab)
61	Design Circular Queue: A Fixed-Capacity Ring Buffer	Queue	Medium	coding	LeetCode (opens in a new tab)
62	Best Time to Buy and Sell Stock: Maximum Profit From One Trade	Sliding Window	Easy	coding	LeetCode (opens in a new tab)
63	Max Sum Subarray of Size K: The Fixed-Size Sliding Window	Sliding Window	Easy	coding	GeeksforGeeks (opens in a new tab)
64	Longest Substring Without Repeating Characters: Sliding Window	Sliding Window	Medium	coding	LeetCode (opens in a new tab)
65	Longest Repeating Character Replacement: Window With At Most k Changes	Sliding Window	Medium	coding	LeetCode (opens in a new tab)
66	Permutation in String: Does One String Contain a Rearrangement of Another?	Sliding Window	Medium	coding	LeetCode (opens in a new tab)
67	Minimum Size Subarray Sum: Grow Until Valid, Then Shrink	Sliding Window	Medium	coding	LeetCode (opens in a new tab)
68	Longest K Unique Characters Substring: A Window Bounded by a Distinct Count	Sliding Window	Medium	coding	GeeksforGeeks (opens in a new tab)
69	Fruit Into Baskets: Longest Window With at Most Two Distinct Values	Sliding Window	Medium	coding	LeetCode (opens in a new tab)
70	Max Consecutive Ones III: Longest Window With at Most K Zeros	Sliding Window	Medium	coding	LeetCode (opens in a new tab)
71	Find All Anagrams in a String: Fixed Window With Letter Counts	Sliding Window	Medium	coding	LeetCode (opens in a new tab)
72	Minimum Window Substring: Shortest Slice Containing Every Required Character	Sliding Window	Hard	coding	LeetCode (opens in a new tab)
73	Sliding Window Maximum: Max of Every Window With a Monotonic Deque	Sliding Window	Hard	coding	LeetCode (opens in a new tab)
74	Substring with Concatenation of All Words: Word-Sized Sliding Windows per Offset	Sliding Window	Hard	coding	LeetCode (opens in a new tab)
75	Valid Parentheses: Check That Every Bracket Closes in the Right Order	Stack	Easy	coding	LeetCode (opens in a new tab)
76	Remove All Adjacent Duplicates In String: Cancel Pairs With a Stack	Stack	Easy	coding	LeetCode (opens in a new tab)
77	Min Stack: A Stack That Returns Its Minimum in Constant Time	Stack	Medium	coding	LeetCode (opens in a new tab)
78	Evaluate Reverse Polish Notation: Compute a Postfix Expression	Stack	Medium	coding	LeetCode (opens in a new tab)
79	Remove All Adjacent Duplicates in String II: Stack of Character Counts	Stack	Medium	coding	LeetCode (opens in a new tab)
80	Simplify Path: Canonical Unix Paths With a Stack of Directory Names	Stack	Medium	coding	LeetCode (opens in a new tab)
81	Next Greater Element I: First Larger Value to the Right	Stack	Easy	coding	LeetCode (opens in a new tab)
82	Daily Temperatures: Days Until a Warmer Day With a Monotonic Stack	Stack	Medium	coding	LeetCode (opens in a new tab)
83	Car Fleet: Count the Groups of Cars Arriving at a Destination	Stack	Medium	coding	LeetCode (opens in a new tab)
84	Next Greater Element II: Monotonic Stack Over a Circular Array	Stack	Medium	coding	LeetCode (opens in a new tab)
85	Remove Nodes From Linked List: Keep Nodes With Nothing Larger to Their Right	Stack	Medium	coding	LeetCode (opens in a new tab)
86	Remove K Digits: Smallest Number With a Greedy Monotonic Stack	Stack	Medium	coding	LeetCode (opens in a new tab)
87	132 Pattern: Find a Low, High, Middle Subsequence in One Pass	Stack	Medium	coding	LeetCode (opens in a new tab)
88	Largest Rectangle in Histogram: Biggest Area Under the Bars	Stack	Hard	coding	LeetCode (opens in a new tab)
89	Binary Search: Find a Target in a Sorted Array in O(log n)	Binary Search	Easy	coding	LeetCode (opens in a new tab)
90	Ceil in Sorted Array: Lower-Bound Binary Search	Binary Search	Easy	coding	GeeksforGeeks (opens in a new tab)
91	Time Based Key-Value Store: Versioned Lookups with Binary Search	Binary Search	Medium	coding, architecture	LeetCode (opens in a new tab)
92	Find First and Last Position of a Value: Lower and Upper Bound Binary Search	Binary Search	Medium	coding	LeetCode (opens in a new tab)
93	Number of Occurrence: Count a Value with Two Boundary Searches	Binary Search	Easy	coding	GeeksforGeeks (opens in a new tab)
94	Median of Two Sorted Arrays: Partition Binary Search in O(log min(m, n))	Binary Search	Hard	coding	LeetCode (opens in a new tab)
95	Find Rotation Count: The Index of the Minimum Is the Answer	Binary Search	Easy	coding	GeeksforGeeks (opens in a new tab)
96	Find Minimum in Rotated Sorted Array: Binary Search Against the Right End	Binary Search	Medium	coding	LeetCode (opens in a new tab)
97	Search in Rotated Sorted Array: One-Pass Binary Search	Binary Search	Medium	coding	LeetCode (opens in a new tab)
98	Peak Index in a Mountain Array: Binary Search on the Slope	Binary Search	Medium	coding	LeetCode (opens in a new tab)
99	Find Peak Element: Follow the Uphill Side with Binary Search	Binary Search	Medium	coding	LeetCode (opens in a new tab)
100	Koko Eating Bananas: Binary Search on the Answer	Binary Search	Medium	coding	LeetCode (opens in a new tab)
101	Minimum Number of Days to Make m Bouquets: Binary Search on the Day	Binary Search	Medium	coding	LeetCode (opens in a new tab)
102	Aggressive Cows: Maximise the Minimum Gap with Binary Search	Binary Search	Medium	coding	GeeksforGeeks (opens in a new tab)
103	Maximum Candies Allocated to K Children: Binary Search on the Share	Binary Search	Medium	coding	LeetCode (opens in a new tab)
104	Capacity To Ship Packages Within D Days: Binary Search on Capacity	Binary Search	Medium	coding	LeetCode (opens in a new tab)
105	Split Array Largest Sum: Binary Search on the Largest Part	Binary Search	Hard	coding	LeetCode (opens in a new tab)
106	Search a 2D Matrix: Binary Search over a Flattened Sorted Grid	Binary Search	Medium	coding	LeetCode (opens in a new tab)
107	Search a 2D Matrix II: Staircase Search from the Top-Right Corner	Binary Search	Medium	coding	LeetCode (opens in a new tab)
108	Kth Smallest Element in a Sorted Matrix: Binary Search on Values	Binary Search	Medium	coding	LeetCode (opens in a new tab)
109	Merge Two Sorted Lists: Dummy Head Iteration and Recursion	Linked List	Easy	coding	LeetCode (opens in a new tab)
110	Remove Nth Node From End of List: One Pass with a Gap of n	Linked List	Medium	coding	LeetCode (opens in a new tab)
111	Copy List with Random Pointer: Hash Map and Interleaving Solutions	Linked List	Medium	coding	LeetCode (opens in a new tab)
112	Add Two Numbers: Digit-by-Digit Addition on Linked Lists	Linked List	Medium	coding	LeetCode (opens in a new tab)
113	LRU Cache: Hash Map Plus Doubly Linked List for O(1) Operations	Linked List	Medium	coding, architecture	LeetCode (opens in a new tab)
114	Linked List Cycle: Floyd's Fast and Slow Pointers	Linked List	Easy	coding	LeetCode (opens in a new tab)
115	Happy Number: Detect a Cycle in a Digit-Square Sequence	Linked List	Easy	coding	LeetCode (opens in a new tab)
116	Middle of the Linked List: Find It in One Pass With Fast and Slow Pointers	Linked List	Easy	coding	LeetCode (opens in a new tab)
117	Palindrome Linked List: Find the Middle, Reverse the Back Half, Compare	Linked List	Easy	coding	LeetCode (opens in a new tab)
118	Reorder List: Find the Middle, Reverse, Then Interleave	Linked List	Medium	coding	LeetCode (opens in a new tab)
119	Find the Duplicate Number: Cycle Detection on an Array	Linked List	Medium	coding	LeetCode (opens in a new tab)
120	Linked List Cycle II: Find Where the Cycle Starts With Floyd's Algorithm	Linked List	Medium	coding	LeetCode (opens in a new tab)
121	Circular Array Loop: Fast and Slow Pointers on Index Jumps	Linked List	Medium	coding	LeetCode (opens in a new tab)
122	Reverse Linked List: Iterative and Recursive Solutions	Linked List	Easy	coding	LeetCode (opens in a new tab)
123	Reverse Linked List II: Reverse a Sublist in One Pass With a Dummy Node	Linked List	Medium	coding	LeetCode (opens in a new tab)
124	Swap Nodes in Pairs: Rewire Every Two Nodes With a Dummy Head	Linked List	Medium	coding	LeetCode (opens in a new tab)
125	Rotate List: Close the Ring, Then Cut It at the New Tail	Linked List	Medium	coding	LeetCode (opens in a new tab)
126	Reverse Nodes in k-Group: In-Place Group Reversal on a Linked List	Linked List	Hard	coding	LeetCode (opens in a new tab)
127	Invert Binary Tree: Recursive and Iterative Mirror Solutions	Trees	Easy	coding	LeetCode (opens in a new tab)
128	Maximum Depth of Binary Tree: Recursive DFS, Iterative DFS and BFS	Trees	Easy	coding	LeetCode (opens in a new tab)
129	Diameter of Binary Tree: Longest Path via Post-Order Heights	Trees	Easy	coding	LeetCode (opens in a new tab)
130	Balanced Binary Tree: Height Check with Early Exit	Trees	Easy	coding	LeetCode (opens in a new tab)
131	Same Tree: Comparing Two Binary Trees Recursively and Iteratively	Trees	Easy	coding	LeetCode (opens in a new tab)
132	Subtree of Another Tree: Brute-Force Matching and Linear Serialisation	Trees	Easy	coding	LeetCode (opens in a new tab)
133	Count Good Nodes in Binary Tree: DFS Carrying the Path Maximum	Trees	Medium	coding	LeetCode (opens in a new tab)
134	Construct Binary Tree from Preorder and Inorder Traversal	Trees	Medium	coding	LeetCode (opens in a new tab)
135	Lowest Common Ancestor of a Binary Tree: One Post-Order Search	Trees	Medium	coding	LeetCode (opens in a new tab)
136	Binary Tree Maximum Path Sum: Post-Order Gains with a Global Best	Trees	Hard	coding	LeetCode (opens in a new tab)
137	Serialize and Deserialize Binary Tree: Preorder with Null Markers	Trees	Hard	coding	LeetCode (opens in a new tab)
138	Binary Tree Level Order Traversal: BFS with a Queue, Level by Level	Trees	Medium	coding	LeetCode (opens in a new tab)
139	Binary Tree Right Side View: Last Node of Each Level	Trees	Medium	coding	LeetCode (opens in a new tab)
140	Implement Trie (Prefix Tree): Insert, Search and Prefix Lookup	Trees	Medium	coding, architecture	LeetCode (opens in a new tab)
141	Design Add and Search Words: Trie Search with Wildcards	Trees	Medium	coding, architecture	LeetCode (opens in a new tab)
142	Extra Characters in a String: Dynamic Programming over a Trie of Words	Trees	Medium	coding	LeetCode (opens in a new tab)
143	Search Suggestions System: Autocomplete with a Trie or Sorted Binary Search	Trees	Medium	coding	LeetCode (opens in a new tab)
144	Word Search II: Find Many Words in a Grid with a Trie and Backtracking	Trees	Hard	coding	LeetCode (opens in a new tab)
145	Convert Sorted Array to BST: Pick the Middle, Recurse on Halves	BST	Easy	coding	LeetCode (opens in a new tab)
146	Validate Binary Search Tree: Bounds Recursion and Inorder Check	BST	Medium	coding	LeetCode (opens in a new tab)
147	Kth Smallest Element in a BST: Inorder Traversal with Early Stop	BST	Medium	coding	LeetCode (opens in a new tab)
148	Lowest Common Ancestor of a BST: Follow the Split Point	BST	Medium	coding	LeetCode (opens in a new tab)
149	Insert into a BST: Walk Down to the Empty Spot	BST	Medium	coding	LeetCode (opens in a new tab)
150	Delete Node in a BST: Leaf, One Child and Two Children Cases	BST	Medium	coding	LeetCode (opens in a new tab)
151	Kth Largest Element in a Stream: Size-k Min-Heap	Heap	Easy	coding	LeetCode (opens in a new tab)
152	Last Stone Weight: Simulating Smashes with a Max-Heap	Heap	Easy	coding	LeetCode (opens in a new tab)
153	Top K Frequent Elements: Return the k Most Common Values	Heap	Medium	coding	LeetCode (opens in a new tab)
154	K Closest Points to Origin: Bounded Max-Heap and Quickselect	Heap	Medium	coding	LeetCode (opens in a new tab)
155	Kth Largest Element in an Array: Heap, Quickselect and Counting	Heap	Medium	coding	LeetCode (opens in a new tab)
156	Task Scheduler: Cooldowns with a Max-Heap or a Counting Formula	Heap	Medium	coding	LeetCode (opens in a new tab)
157	Min Cost to Connect Ropes: Always Join the Two Shortest	Heap	Medium	coding	GeeksforGeeks (opens in a new tab)
158	Sort Characters By Frequency: Count, Then Order by a Heap or Buckets	Heap	Medium	coding	LeetCode (opens in a new tab)
159	Find K Closest Elements: Binary Search for the Best Window	Heap	Medium	coding	LeetCode (opens in a new tab)
160	Reorganize String: Place the Most Frequent Character First, Never Twice in a Row	Heap	Medium	coding	LeetCode (opens in a new tab)
161	Maximum Frequency Stack: Pop the Most Frequent Value in O(1)	Heap	Hard	coding, architecture	LeetCode (opens in a new tab)
162	Maximum Sum Combination: The K Largest Pair Sums with a Max-Heap	Heap	Medium	coding	GeeksforGeeks (opens in a new tab)
163	Find Median from Data Stream: Two Heaps Kept in Balance	Heap	Hard	coding	LeetCode (opens in a new tab)
164	Sliding Window Median: Two Heaps with Lazy Deletion	Heap	Hard	coding	LeetCode (opens in a new tab)
165	IPO: Maximise Capital by Picking the Best Affordable Project Each Round	Heap	Hard	coding	LeetCode (opens in a new tab)
166	Design Twitter: News Feed with a K-Way Heap Merge	Heap	Medium	coding, architecture	LeetCode (opens in a new tab)
167	Merge k Sorted Lists: Min-Heap and Divide-and-Conquer Solutions	Heap	Hard	coding	LeetCode (opens in a new tab)
168	Smallest Range Covering Elements from K Lists: K-Way Merge with a Min-Heap	Heap	Hard	coding	LeetCode (opens in a new tab)
169	Jump Game: Can You Reach the Last Index? Greedy Furthest Reach	Greedy	Medium	coding	LeetCode (opens in a new tab)
170	Jump Game II: Fewest Jumps to the End with Greedy Level-by-Level BFS	Greedy	Medium	coding	LeetCode (opens in a new tab)
171	Gas Station: Find the Start of a Circular Route in One Greedy Pass	Greedy	Medium	coding	LeetCode (opens in a new tab)
172	Hand of Straights: Group Cards into Consecutive Runs Greedily	Greedy	Medium	coding	LeetCode (opens in a new tab)
173	Merge Triplets to Form Target: Keep Only Triplets That Never Overshoot	Greedy	Medium	coding	LeetCode (opens in a new tab)
174	Partition Labels: Split a String So Each Letter Stays in One Part	Greedy	Medium	coding	LeetCode (opens in a new tab)
175	Valid Parenthesis String: Track a Range of Open Counts Greedily	Greedy	Medium	coding	LeetCode (opens in a new tab)
176	Maximum Length of Pair Chain: Greedy by Earliest End	Greedy	Medium	coding	LeetCode (opens in a new tab)
177	Minimum Add to Make Parentheses Valid: Count Unmatched Brackets in One Pass	Greedy	Medium	coding	LeetCode (opens in a new tab)
178	Remove Duplicate Letters: Smallest Order with a Greedy Monotonic Stack	Greedy	Medium	coding	LeetCode (opens in a new tab)
179	Meeting Rooms: Check Whether One Person Can Attend Every Meeting	Greedy	Easy	coding	GeeksforGeeks (opens in a new tab)
180	Merge Intervals: Combine Every Overlapping Range	Greedy	Medium	coding	LeetCode (opens in a new tab)
181	Insert Interval: Add a Range to a Sorted List and Merge Overlaps	Greedy	Medium	coding	LeetCode (opens in a new tab)
182	Non-overlapping Intervals: Fewest Removals to Eliminate Overlaps	Greedy	Medium	coding	LeetCode (opens in a new tab)
183	Interval List Intersections: Two Pointers over Sorted Ranges	Greedy	Medium	coding	LeetCode (opens in a new tab)
184	Meeting Rooms II: Count the Rooms with a Min-Heap or a Sweep Line	Greedy	Medium	coding	GeeksforGeeks (opens in a new tab)
185	My Calendar I: Accept Bookings Only When They Do Not Overlap	Greedy	Medium	coding	LeetCode (opens in a new tab)
186	Generate Parentheses: List Every Balanced String of n Pairs	Backtracking	Medium	coding	LeetCode (opens in a new tab)
187	Subsets: Generate the Power Set with Include/Exclude Backtracking	Backtracking	Medium	coding	LeetCode (opens in a new tab)
188	Permutations: Every Ordering of Distinct Values with Backtracking	Backtracking	Medium	coding	LeetCode (opens in a new tab)
189	Subsets II: Power Set Without Duplicates When Values Repeat	Backtracking	Medium	coding	LeetCode (opens in a new tab)
190	Letter Combinations of a Phone Number: Cartesian Product by Backtracking	Backtracking	Medium	coding	LeetCode (opens in a new tab)
191	Letter Case Permutation: Branch Twice on Every Letter	Backtracking	Medium	coding	LeetCode (opens in a new tab)
192	Combination Sum: Reach a Target with Reusable Values via Backtracking	Backtracking	Medium	coding	LeetCode (opens in a new tab)
193	Combination Sum II: Use Each Value Once and Avoid Duplicate Answers	Backtracking	Medium	coding	LeetCode (opens in a new tab)
194	Word Search: Trace a Word Through a Letter Grid with DFS Backtracking	Backtracking	Medium	coding	LeetCode (opens in a new tab)
195	Palindrome Partitioning: Split a String into Palindromes by Backtracking	Backtracking	Medium	coding	LeetCode (opens in a new tab)
196	N-Queens: Place Non-Attacking Queens Row by Row with Backtracking	Backtracking	Hard	coding	LeetCode (opens in a new tab)
197	Sudoku Solver: Backtracking with Constraint Sets and Smart Cell Choice	Backtracking	Hard	coding	LeetCode (opens in a new tab)
198	Find if Path Exists in Graph: Reachability with BFS or Union-Find	Graph	Easy	coding	LeetCode (opens in a new tab)
199	Clone Graph: Deep-Copy a Connected Graph with a Hash Map and BFS or DFS	Graph	Medium	coding	LeetCode (opens in a new tab)
200	Number of Provinces: Count Components in an Adjacency Matrix	Graph	Medium	coding	LeetCode (opens in a new tab)
201	Word Ladder: Shortest Word Transformation with BFS and Wildcard Buckets	Graph	Hard	coding	LeetCode (opens in a new tab)
202	Reconstruct Itinerary: Eulerian Path with Hierholzer's Algorithm	Graph	Hard	coding	LeetCode (opens in a new tab)
203	Flood Fill: Repaint a Connected Region with DFS or BFS	Graph	Easy	coding	LeetCode (opens in a new tab)
204	Island Perimeter: Count Land Edges Without a Traversal	Graph	Easy	coding	LeetCode (opens in a new tab)
205	Number of Islands: Count Connected Land Cells with BFS, DFS or Union-Find	Graph	Medium	coding	LeetCode (opens in a new tab)
206	Max Area of Island: Largest Connected Land Region with Flood Fill	Graph	Medium	coding	LeetCode (opens in a new tab)
207	Pacific Atlantic Water Flow: Reverse Multi-Source BFS from Both Oceans	Graph	Medium	coding	LeetCode (opens in a new tab)
208	Surrounded Regions: Capture Enclosed Cells by Flood-Filling from the Border	Graph	Medium	coding	LeetCode (opens in a new tab)
209	Rotting Oranges: Minutes to Spread via Level-by-Level Multi-Source BFS	Graph	Medium	coding	LeetCode (opens in a new tab)
210	Number of Closed Islands: Sink the Border, Then Count	Graph	Medium	coding	LeetCode (opens in a new tab)
211	Course Schedule: Detect a Cycle in a Prerequisite Graph with Kahn's Algorithm	Graph	Medium	coding	LeetCode (opens in a new tab)
212	Course Schedule II: Return a Valid Course Order with Topological Sort	Graph	Medium	coding	LeetCode (opens in a new tab)
213	Topological Sort: Order a DAG with Kahn's Algorithm or DFS	Graph	Medium	coding	GeeksforGeeks (opens in a new tab)
214	Minimum Height Trees: Trim Leaves Until the Centre Remains	Graph	Medium	coding	LeetCode (opens in a new tab)
215	Alien Dictionary: Derive a Letter Order from Sorted Words with Topological Sort	Graph	Hard	coding	GeeksforGeeks (opens in a new tab)
216	Graph Valid Tree: Check Edge Count, Connectivity and Cycles with Union-Find	Graph	Medium	coding	GeeksforGeeks (opens in a new tab)
217	Number of Connected Components in an Undirected Graph with Union-Find	Graph	Medium	coding	GeeksforGeeks (opens in a new tab)
218	Redundant Connection: Find the Edge That Closes a Cycle with Union-Find	Graph	Medium	coding	LeetCode (opens in a new tab)
219	Is Graph Bipartite?: Two-Colour Every Component	Graph	Medium	coding	LeetCode (opens in a new tab)
220	Path With Minimum Effort: Minimise the Largest Step on a Grid	Graph	Medium	coding	LeetCode (opens in a new tab)
221	Network Delay Time: Single-Source Shortest Paths with Dijkstra's Algorithm	Graph	Medium	coding	LeetCode (opens in a new tab)
222	Cheapest Flights Within K Stops: Bounded Bellman-Ford for Limited Hops	Graph	Medium	coding	LeetCode (opens in a new tab)
223	Min Cost to Connect All Points: Minimum Spanning Tree with Prim or Kruskal	Graph	Medium	coding	LeetCode (opens in a new tab)
224	Swim in Rising Water: Minimax Path with a Modified Dijkstra	Graph	Hard	coding	LeetCode (opens in a new tab)
225	Climbing Stairs: Count Ways with a Fibonacci-Style DP	Dynamic Programming	Easy	coding	LeetCode (opens in a new tab)
226	Min Cost Climbing Stairs: Cheapest Path to the Top with 1D DP	Dynamic Programming	Easy	coding	LeetCode (opens in a new tab)
227	House Robber: Maximum Non-Adjacent Sum with Take-or-Skip DP	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
228	House Robber II: Non-Adjacent Maximum Sum When Houses Form a Circle	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
229	Decode Ways: Count Digit-to-Letter Decodings with Prefix DP	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
230	Coin Change: Fewest Coins with Bottom-Up DP	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
231	Word Break: Can a String Be Split into Dictionary Words? Prefix DP	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
232	Longest Increasing Subsequence: O(n^2) DP and O(n log n) Patience Sorting	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
233	Best Time to Buy and Sell Stock with Cooldown: State-Machine DP	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
234	Partition Equal Subset Sum: 0/1 Knapsack on Half the Total	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
235	Coin Change II: Count Combinations That Make an Amount	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
236	Target Sum: Count Sign Assignments with Subset-Sum DP	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
237	0 - 1 Knapsack Problem: Take or Skip Each Item Once	Dynamic Programming	Medium	coding	GeeksforGeeks (opens in a new tab)
238	Subset Sum Problem: Track Every Reachable Total	Dynamic Programming	Medium	coding	GeeksforGeeks (opens in a new tab)
239	Count Subsets with Sum: Add Up the Ways, Not Just True or False	Dynamic Programming	Medium	coding	GeeksforGeeks (opens in a new tab)
240	Partition Into 2 Subsets with Min Sum Diff: Subset Sum Up to Half the Total	Dynamic Programming	Hard	coding	GeeksforGeeks (opens in a new tab)
241	Longest Palindromic Substring: Expand Around Centres or Interval DP	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
242	Palindromic Substrings: Count Every Palindrome by Expanding Around Centres	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
243	Unique Paths: Count Grid Routes with 2D DP or a Binomial Coefficient	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
244	Longest Common Subsequence: The Classic Two-String DP Table	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
245	Interleaving String: Can Two Strings Merge into a Third? 2D DP	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
246	Edit Distance: Levenshtein Distance with a 2D DP Table	Dynamic Programming	Medium	coding	LeetCode (opens in a new tab)
247	Burst Balloons: Interval DP by Choosing the Last Balloon to Burst	Dynamic Programming	Hard	coding	LeetCode (opens in a new tab)
248	Regular Expression Matching: Dot and Star with a 2D DP Table	Dynamic Programming	Hard	coding	LeetCode (opens in a new tab)

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