Most-asked coding interview problems

The 50 problems tagged by the most companies in our interview dataset. Each page lets you write and test a solution in Python or JavaScript in your browser, then walks through the approach from brute force to optimal.

Problem	Pattern	Difficulty	Companies
Two Sum	Hash map of complements	Easy	196
LRU Cache	Hash map + doubly linked list	Medium	196
Number of Islands	Flood fill (DFS/BFS on a grid)	Medium	189
Merge Intervals	Sort, then sweep	Medium	183
Valid Parentheses	Stack of open brackets	Easy	180
Longest Palindromic Substring	Expand around center	Medium	166
Longest Substring Without Repeating Characters	Sliding window with last-seen index	Medium	165
Best Time to Buy and Sell Stock	Running minimum	Easy	163
Trapping Rain Water	Two pointers with running maximums	Hard	161
Group Anagrams	Hash map keyed by a canonical form	Medium	156
Container With Most Water	Two pointers from both ends	Medium	153
Maximum Subarray	Kadane's algorithm	Medium	149
Search in Rotated Sorted Array	Binary search on the sorted half	Medium	149
Spiral Matrix	Shrinking boundaries	Medium	143
Rotting Oranges	Multi-source BFS	Medium	142
String Compression	Read and write pointers	Medium	140
Valid Palindrome	Two pointers from both ends	Easy	137
Merge k Sorted Lists	Min-heap of list heads	Hard	135
Top K Frequent Elements	Count, then bucket by frequency	Medium	135
Product of Array Except Self	Prefix and suffix products	Medium	133
Rotate Image	Transpose, then reverse each row	Medium	133
Roman to Integer	Compare each symbol with the next	Easy	133
Course Schedule	Topological sort (Kahn’s algorithm)	Medium	132
Decode String	Stack for nested structure	Medium	132
Sliding Window Maximum	Monotonic deque	Hard	131
Climbing Stairs	1D dynamic programming	Easy	130
Merge Sorted Array	Two pointers from the back	Easy	129
3Sum	Sort + two pointers	Medium	128
Median of Two Sorted Arrays	Binary search on a partition	Hard	127
Basic Calculator II	Running term with deferred addition	Medium	127
Longest Consecutive Sequence	Hash set, count from run starts	Medium	126
Word Ladder	Breadth-first search on an implicit graph	Hard	126
Next Permutation	Rightmost ascent, swap, reverse	Medium	126
Add Two Numbers	Digit-by-digit addition with a carry	Medium	125
Longest Common Prefix	Vertical scan	Easy	124
Letter Combinations of a Phone Number	Backtracking over choices	Medium	124
Subarray Sum Equals K	Prefix sums + hash map of counts	Medium	123
Generate Parentheses	Backtracking with constraints	Medium	123
Coin Change	Bottom-up DP over amounts	Medium	123
Gas Station	Greedy restart on deficit	Medium	123
Kth Largest Element in an Array	Quickselect	Medium	122
Minimum Window Substring	Variable-size sliding window with counts	Hard	120
Min Stack	Stack with running minimum	Medium	120
Palindrome Number	Reverse half the digits	Easy	118
House Robber	1D dynamic programming	Medium	118
Basic Calculator	Running sum with a stack of saved contexts	Hard	118
Word Search	Grid backtracking (DFS)	Medium	117
Subsets II	Backtracking with duplicate skipping	Medium	117
Move Zeroes	Read/write two pointers	Easy	116
All Nodes Distance K in Binary Tree	Tree to graph + BFS	Medium	116

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Most-asked coding interview problems

Problem	Pattern	Difficulty	Companies
Two Sum	Hash map of complements	Easy	196
LRU Cache	Hash map + doubly linked list	Medium	196
Number of Islands	Flood fill (DFS/BFS on a grid)	Medium	189
Merge Intervals	Sort, then sweep	Medium	183
Valid Parentheses	Stack of open brackets	Easy	180
Longest Palindromic Substring	Expand around center	Medium	166
Longest Substring Without Repeating Characters	Sliding window with last-seen index	Medium	165
Best Time to Buy and Sell Stock	Running minimum	Easy	163
Trapping Rain Water	Two pointers with running maximums	Hard	161
Group Anagrams	Hash map keyed by a canonical form	Medium	156
Container With Most Water	Two pointers from both ends	Medium	153
Maximum Subarray	Kadane's algorithm	Medium	149
Search in Rotated Sorted Array	Binary search on the sorted half	Medium	149
Spiral Matrix	Shrinking boundaries	Medium	143
Rotting Oranges	Multi-source BFS	Medium	142
String Compression	Read and write pointers	Medium	140
Valid Palindrome	Two pointers from both ends	Easy	137
Merge k Sorted Lists	Min-heap of list heads	Hard	135
Top K Frequent Elements	Count, then bucket by frequency	Medium	135
Product of Array Except Self	Prefix and suffix products	Medium	133
Rotate Image	Transpose, then reverse each row	Medium	133
Roman to Integer	Compare each symbol with the next	Easy	133
Course Schedule	Topological sort (Kahn’s algorithm)	Medium	132
Decode String	Stack for nested structure	Medium	132
Sliding Window Maximum	Monotonic deque	Hard	131
Climbing Stairs	1D dynamic programming	Easy	130
Merge Sorted Array	Two pointers from the back	Easy	129
3Sum	Sort + two pointers	Medium	128
Median of Two Sorted Arrays	Binary search on a partition	Hard	127
Basic Calculator II	Running term with deferred addition	Medium	127
Longest Consecutive Sequence	Hash set, count from run starts	Medium	126
Word Ladder	Breadth-first search on an implicit graph	Hard	126
Next Permutation	Rightmost ascent, swap, reverse	Medium	126
Add Two Numbers	Digit-by-digit addition with a carry	Medium	125
Longest Common Prefix	Vertical scan	Easy	124
Letter Combinations of a Phone Number	Backtracking over choices	Medium	124
Subarray Sum Equals K	Prefix sums + hash map of counts	Medium	123
Generate Parentheses	Backtracking with constraints	Medium	123
Coin Change	Bottom-up DP over amounts	Medium	123
Gas Station	Greedy restart on deficit	Medium	123
Kth Largest Element in an Array	Quickselect	Medium	122
Minimum Window Substring	Variable-size sliding window with counts	Hard	120
Min Stack	Stack with running minimum	Medium	120
Palindrome Number	Reverse half the digits	Easy	118
House Robber	1D dynamic programming	Medium	118
Basic Calculator	Running sum with a stack of saved contexts	Hard	118
Word Search	Grid backtracking (DFS)	Medium	117
Subsets II	Backtracking with duplicate skipping	Medium	117
Move Zeroes	Read/write two pointers	Easy	116
All Nodes Distance K in Binary Tree	Tree to graph + BFS	Medium	116

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