Algorithm problems
53 problems for the coding round of IT interviews — frequently asked ones first, each tagged by topic so you can revise group by group.
Results
Sorted by how many companies ask them
Easy 15 min
Best Time to Buy and Sell Stock
GreedyEasy 15 min
Invert Binary Tree
DFS (Depth-First Search)Easy 20 min
Merge Two Sorted Lists
Linked ListEasy 15 min
Reverse Linked List
Linked ListEasy 15 min
Two Sum
Hash MapEasy 12 min
Valid Anagram
Hash MapEasy 15 min
Valid Parentheses
StackMedium 25 min
Binary Tree Level Order Traversal
BFS (Breadth-First Search)Medium 30 min
Combination Sum
BacktrackingMedium 30 min
Course Schedule
DFS (Depth-First Search)Medium 25 min
Group Anagrams
Hash MapMedium 25 min
House Robber
1D Dynamic ProgrammingMedium 30 min
Longest Repeating Character Replacement
Sliding WindowMedium 25 min
Longest Substring Without Repeating Characters
Sliding WindowMedium 20 min
Maximum Subarray
GreedyMedium 25 min
Merge Intervals
IntervalsMedium 25 min
Min Stack
StackMedium 25 min
Number of Islands
BFS (Breadth-First Search)Medium 25 min
Search in Rotated Sorted Array
Binary SearchMedium 30 min
3Sum
Two PointersMedium 25 min
Top K Frequent Elements
Hash MapMedium 15 min
Two Sum II - Input Array Is Sorted
Two PointersEasy 12 min
Binary Search
Binary SearchEasy 15 min
Climbing Stairs
1D Dynamic ProgrammingEasy 10 min
Contains Duplicate
Hash MapEasy 20 min
Linked List Cycle
Linked ListEasy 15 min
Maximum Subarray of Size K
Sliding WindowEasy 15 min
Meeting Rooms
IntervalsEasy 15 min
Search Insert Position
Binary SearchEasy 12 min
Valid Palindrome
Two PointersMedium 25 min
Clone Graph
DFS (Depth-First Search)Medium 20 min
Container With Most Water
Two PointersMedium 20 min
Daily Temperatures
StackMedium 18 min
Evaluate Reverse Polish Notation
StackMedium 20 min
Find Minimum in Rotated Sorted Array
Binary SearchMedium 24 min
Gas Station
GreedyMedium 30 min
Insert Interval
IntervalsMedium 25 min
Jump Game
GreedyMedium 35 min
Longest Increasing Subsequence
1D Dynamic ProgrammingMedium 22 min
Non-overlapping Intervals
IntervalsMedium 25 min
Number of Connected Components
DFS (Depth-First Search)Medium 25 min
Permutation in String
Sliding WindowMedium 25 min
Permutations
BacktrackingMedium 25 min
Subsets
BacktrackingMedium 30 min
Word Break
1D Dynamic ProgrammingHard 40 min
Word Ladder
BFS (Breadth-First Search)Easy 15 min
Range Sum Query - Immutable
Prefix SumMedium 20 min
Kth Largest Element in an Array
HeapMedium 20 min
Product of Array Except Self
Prefix SumMedium 25 min
Subarray Sum Equals K
Prefix SumHard 30 min
Find Median from Data Stream
HeapHard 30 min
Merge K Sorted Lists
HeapHard 30 min