Most-asked coding interview problems

The 100 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
Add Strings	Digit-by-digit addition with a carry	Easy	87
Sqrt(x)	Binary search on the answer	Easy	103
Minimum Array Sum	0/1 knapsack DP with two budgets	Medium	87
Meeting Rooms II	Sweep line over start and end events	Medium	117
Daily Temperatures	Monotonic decreasing stack	Medium	106
Minimum Path Sum	2D grid dynamic programming	Medium	104
Ransom Note	Character frequency counting	Easy	99
Valid Anagram	Character frequency counting	Easy	110
Missing Ranges	Linear scan with sentinels	Easy	116
Isomorphic Strings	Bijection check with two hash maps	Easy	82
Logger Rate Limiter	Hash map of next allowed time	Easy	101
Pascal's Triangle	Build each row from the previous row	Easy	96
Merge Strings Alternately	Two pointers over two sequences	Easy	102
Reverse Words in a String	Tokenize, then reverse	Medium	102
Length of Last Word	Scan from the end	Easy	75
Find the Index of the First Occurrence in a String	String matching with a prefix function (KMP)	Easy	111
Fraction to Recurring Decimal	Long division with a remainder map	Medium	90
Remove Duplicates from Sorted Array	Slow/fast two pointers	Easy	109
Analyze User Website Visit Pattern	Group, enumerate triples, count with a set per user	Medium	95
Maximum Number of Occurrences of a Substring	Fixed-size sliding window with a frequency map	Medium	90
Invalid Transactions	Parse, group by key, compare within groups	Medium	75
Custom Sort String	Counting sort by a custom key	Medium	74
Accounts Merge	Union-find over shared keys	Medium	108
Expression Add Operators	Backtracking with a running value	Hard	99
Combination Sum	Backtracking with a start index	Medium	111
Maximum Profit in Job Scheduling	Sort by end + DP + binary search	Hard	99
Sum Root to Leaf Numbers	DFS with a running prefix value	Medium	89
Closest Binary Search Tree Value	Walk one BST search path	Easy	89
Minimum Limit of Balls in a Bag	Binary search on the answer	Medium	95
Subarrays with K Different Integers	Sliding window: exactly k = at most k - at most (k - 1)	Hard	116
Squares of a Sorted Array	Two pointers from both ends	Easy	107
Missing Number	Sum or XOR identity	Easy	85
Maximize Amount After Two Days of Conversions	Graph traversal with multiplied edge weights	Medium	92
Binary Tree Right Side View	Level-order traversal (BFS by level)	Medium	93
Word Break II	Backtracking with memoization	Hard	98
Valid Palindrome II	Two pointers with one branch	Easy	94
Flood Fill	Grid DFS/BFS over a connected component	Easy	106
Maximum Matrix Sum	Parity invariant (greedy)	Medium	92
Word Ladder II	BFS layers + backtracking over a parent DAG	Hard	97
Counting Bits	DP over bits	Easy	93
Search Suggestions System	Sort, then binary search each prefix	Medium	110
Remove All Adjacent Duplicates in String II	Stack of (character, run length)	Medium	89
Ways to Make a Fair Array	Prefix sums split by parity	Medium	84
Can Place Flowers	Greedy left-to-right placement	Easy	86
Longest Arithmetic Subsequence of Given Difference	DP in a hash map keyed by value	Medium	89
Longest Repeating Character Replacement	Sliding window with a max-frequency count	Medium	102
Remove All Adjacent Duplicates in String	Stack as the surviving prefix	Easy	93
Palindrome Linked List	Fast/slow pointers plus in-place reversal	Easy	95
Find Peak Element	Binary search on the slope	Medium	102
Check If a Word Occurs As a Prefix of Any Word in a Sentence	Split and check prefixes	Easy	98

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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
Add Strings	Digit-by-digit addition with a carry	Easy	87
Sqrt(x)	Binary search on the answer	Easy	103
Minimum Array Sum	0/1 knapsack DP with two budgets	Medium	87
Meeting Rooms II	Sweep line over start and end events	Medium	117
Daily Temperatures	Monotonic decreasing stack	Medium	106
Minimum Path Sum	2D grid dynamic programming	Medium	104
Ransom Note	Character frequency counting	Easy	99
Valid Anagram	Character frequency counting	Easy	110
Missing Ranges	Linear scan with sentinels	Easy	116
Isomorphic Strings	Bijection check with two hash maps	Easy	82
Logger Rate Limiter	Hash map of next allowed time	Easy	101
Pascal's Triangle	Build each row from the previous row	Easy	96
Merge Strings Alternately	Two pointers over two sequences	Easy	102
Reverse Words in a String	Tokenize, then reverse	Medium	102
Length of Last Word	Scan from the end	Easy	75
Find the Index of the First Occurrence in a String	String matching with a prefix function (KMP)	Easy	111
Fraction to Recurring Decimal	Long division with a remainder map	Medium	90
Remove Duplicates from Sorted Array	Slow/fast two pointers	Easy	109
Analyze User Website Visit Pattern	Group, enumerate triples, count with a set per user	Medium	95
Maximum Number of Occurrences of a Substring	Fixed-size sliding window with a frequency map	Medium	90
Invalid Transactions	Parse, group by key, compare within groups	Medium	75
Custom Sort String	Counting sort by a custom key	Medium	74
Accounts Merge	Union-find over shared keys	Medium	108
Expression Add Operators	Backtracking with a running value	Hard	99
Combination Sum	Backtracking with a start index	Medium	111
Maximum Profit in Job Scheduling	Sort by end + DP + binary search	Hard	99
Sum Root to Leaf Numbers	DFS with a running prefix value	Medium	89
Closest Binary Search Tree Value	Walk one BST search path	Easy	89
Minimum Limit of Balls in a Bag	Binary search on the answer	Medium	95
Subarrays with K Different Integers	Sliding window: exactly k = at most k - at most (k - 1)	Hard	116
Squares of a Sorted Array	Two pointers from both ends	Easy	107
Missing Number	Sum or XOR identity	Easy	85
Maximize Amount After Two Days of Conversions	Graph traversal with multiplied edge weights	Medium	92
Binary Tree Right Side View	Level-order traversal (BFS by level)	Medium	93
Word Break II	Backtracking with memoization	Hard	98
Valid Palindrome II	Two pointers with one branch	Easy	94
Flood Fill	Grid DFS/BFS over a connected component	Easy	106
Maximum Matrix Sum	Parity invariant (greedy)	Medium	92
Word Ladder II	BFS layers + backtracking over a parent DAG	Hard	97
Counting Bits	DP over bits	Easy	93
Search Suggestions System	Sort, then binary search each prefix	Medium	110
Remove All Adjacent Duplicates in String II	Stack of (character, run length)	Medium	89
Ways to Make a Fair Array	Prefix sums split by parity	Medium	84
Can Place Flowers	Greedy left-to-right placement	Easy	86
Longest Arithmetic Subsequence of Given Difference	DP in a hash map keyed by value	Medium	89
Longest Repeating Character Replacement	Sliding window with a max-frequency count	Medium	102
Remove All Adjacent Duplicates in String	Stack as the surviving prefix	Easy	93
Palindrome Linked List	Fast/slow pointers plus in-place reversal	Easy	95
Find Peak Element	Binary search on the slope	Medium	102
Check If a Word Occurs As a Prefix of Any Word in a Sentence	Split and check prefixes	Easy	98

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