Algo Kernel

The core 150 coding problems every candidate should master before a technical interview.

144 problems•20 sections

Difficulty

Problem	Difficulty	Video
Two Sum	easy
Isomorphic Strings	easy
Contains Duplicate 1 - Check if array has duplicates	easy
Valid Anagram	easy
Is Subsequence	easy
Valid Sudoku	medium
Group Anagrams	medium
Longest Consecutive Sequence	medium
Encode and Decode Strings	medium

Problem	Difficulty	Video
Valid Palindrome	easy
Backspace String Compare	easy
Container With Most Water	medium
3Sum0 - Find three elements in an array that sum to a zero	medium
Dutch National Flag DNF	medium
2Sum 2 – Input array is sorted	medium
Trapping Rain Water	hard

Problem	Difficulty	Video
Longest Substring Without Repeating Characters	medium
Longest Substring with Same Letters after Replacement	medium
Find anagrams of a string in another string	medium
Count Number of Nice Subarrays	medium
Minimum Window Substring	hard
Sliding Window Maximum	hard

Problem	Difficulty	Video
Product of Array Except Self	medium

Problem	Difficulty	Video
Kth Largest Element in an Array	medium

Problem	Difficulty	Video
Valid Parentheses	easy
Evaluate Reverse Polish Notation	medium
Min Stack	medium
Daily Temperatures	medium
Car Fleet	medium
Online Stock Span	medium
Largest Rectangle in Histogram	hard

Problem	Difficulty	Video
Binary Search on Sorted Array	easy
Search in Rotated Sorted Array	medium
Search a 2D Matrix 1 - Elements sorted row-wise and first integer of each row is greater than last element of previous row	medium
Find Minimum in Rotated Sorted Array	medium
Koko Eating Bananas	medium	-
Time Based Key-Value Store	medium
Median of Two Sorted Arrays	hard	-

Problem	Difficulty	Video
Merge Two Sorted Linked Lists	easy
Linked List Cycle 1 - Detect Cycle	easy
Reverse Linked List	easy
Add Two Numbers represented as Linked List in Reversed Order	medium
Remove Nth Node From End of List	medium
Copy List with Random Pointer	medium
Reorder List such that i-th element points to n-i-th element	medium
Design LRU Cache	medium
Find Duplicate Number in array containing n+1 numbers between 1 and n	medium	-
K Reverse a linked list	hard	-

Problem	Difficulty	Video
Same Tree	easy
Maximum Depth of Binary Tree	easy
Balanced Binary Tree	easy	-
Invert Binary Tree	easy
Diameter Of a Binary Tree	easy	-
Subtree of Another Tree	easy
Validate Binary Search Tree	medium
Binary Tree Level Order Traversal - Level by Level	medium
Construct Binary Tree from Inorder and Preorder Traversal	medium
Binary Tree Right Side View	medium	-
Kth Smallest Element in a BST	medium
Lowest Common Ancestor of a Binary Search Tree	medium
Binary Tree Path Sum - Maximum between any two nodes	hard
Serialize and Deserialize Binary Tree	hard

Problem	Difficulty	Video
Implement Trie	medium
Design Add and Search Words Data Structure	medium
Word Search 2 - Return All Words	hard

Problem	Difficulty	Video
Kth Largest Element in a Stream	easy
Last Stone Weight	easy	-
Top K Frequent Elements	medium
Design Twitter (Data Structure Design)	medium	-
Task Scheduler	medium	-
K closest Points to Origin	medium	-
Merge K Sorted Lists	hard
Find Median from Data Stream	hard

Problem	Difficulty	Video
Letter Combinations of a Phone Number	medium	-
Combination Sum 1	medium
Combination Sum 2 - Cant Reuse same element	medium	-
Permutations of Array 1	medium	-
Subsets 1	medium
Word Search 1 - Find if word exists	medium
Subsets 2	medium	-
Palindrome Partitioning	medium
N-Queens	hard	-

Problem	Difficulty	Video
Rotting Oranges	medium	-
Clone Undirected Graph	medium
Number of Islands	medium
Course Schedule 1 - Is it Possible	medium
Course Schedule 2 - Get Ordered Courses	medium	-
Graph Valid Tree	medium
Walls and Gates	medium	-
Pacific Atlantic Water Flow	medium
Network Delay Time	medium	-
Cheapest Flights Within K Stops Problem	medium	-
Min Cost to Connect All Points	medium	-
Word Ladder 1 - Get Ladder Length	hard	-
Alien Dictionary	hard
Reconstruct Itinerary	hard	-
Swim in Rising Water	hard	-

Problem	Difficulty	Video
Climbing Stairs - Take atmost 2 Steps	easy
Best Time To Buy And Sell Stock 1 - only one transaction	easy
Min Cost Climbing Stairs	easy	-
Longest Palindromic Substring	medium
Maximum Subarray Sum	medium
Unique Paths in Grid 1 - Count all paths moving right or down	medium
Edit Distance	medium	-
Decode Ways	medium
Interleaving String	medium	-
Word Break 1 - Check if word is breakable	medium
Maximum Product Subarray	medium
House Robber	medium
House Robber 2 - Houses in circle	medium
Longest Increasing Subsequence	medium
Best Time To Buy And Sell Stock With Cooldown	medium	-
Coin Change with Fewest Number of Coins Given Infinite Supply	medium
Partition Equal Subset Sum	medium	-
Target Sum	medium
Palindromic Substrings	medium
Longest Common Subsequence LCS 1 - Get Length	medium
Burst Balloons	hard	-
Longest Increasing Path in a Matrix	hard	-

Problem	Difficulty	Video
Majority Element 1 - with n by 2 definition	easy	-
Jump Game 2 - Get min jumps	medium	-
Jump Game 1 - Check if it reaches last index	medium
Gas Station	medium	-
Valid Parenthesis String	medium	-
Partition Labels	medium	-
Hand of Straights	medium

Problem	Difficulty	Video
Meeting Rooms 1 - Can person attend all meetings	easy
Merge Overlapping Intervals	medium
Insert Interval	medium
Maximum Intervals Overlap Count	medium
Non-overlapping Intervals	medium

Problem	Difficulty	Video
Maximum Points on a Line	hard	-

Problem	Difficulty	Video
Single Number 1 - All elements except one occur twice	easy
Reverse Bits	easy
Number of 1 Bits	easy
Missing Number	easy
Counting Bits	easy
Sum of Two Integers	medium

Problem	Difficulty	Video
Number of Connected Components in an Undirected Graph	medium	-
Redundant Connection	medium	-
Accounts Merge	medium	-

Arrays & Hashing