The Blind 75, visualized.

#1 · Arrays & Hashing

Two Sum

Given an array of integers and a target, return the indices of the two numbers that add up to the target. Exactly one solution exists and you may not use the same element twice.

#2 · Sliding Window

Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring that contains no repeated character.

#3 · Trees

Invert Binary Tree

Given the root of a binary tree, invert it (swap every node's left and right children) and return its root.

#4 · Graphs

Course Schedule

There are n courses, and prerequisites[i] = [a, b] means you must take b before a. Return true if you can finish all courses, which is the same as asking whether the prerequisite graph has no cycle.

#5 · 1D DP

Coin Change

Given coin denominations and a target amount, return the fewest coins needed to make that amount, or −1 if it is impossible. Each coin can be used any number of times.

#6 · Arrays & Hashing

Contains Duplicate

Given an integer array, return true if any value appears at least twice, and false if every element is distinct.

#7 · Arrays & Hashing

Valid Anagram

Given two strings s and t, return true if t is an anagram of s: the same letters, each used exactly the same number of times, in any order.

#8 · Sliding Window

Best Time to Buy and Sell Stock

Given daily prices, choose one day to buy and a later day to sell to maximize profit. Return the maximum profit, or 0 if no profitable trade exists.

#9 · Arrays & Hashing

Product of Array Except Self

Return an array where out[i] is the product of every element of nums except nums[i], in O(n) time and without using division.

#10 · Arrays & Hashing

Group Anagrams

Given an array of strings, group the anagrams together. You may return the groups in any order.

#11 · Arrays & Hashing

Encode and Decode Strings

Design an algorithm to encode a list of strings into a single string, and decode it back to the original list. The strings can contain any characters, so no character is safe to use as a plain separator.

#12 · Arrays & Hashing

Longest Consecutive Sequence

Given an unsorted array of integers, return the length of the longest run of consecutive values (like 1, 2, 3, 4), regardless of their order in the array. The solution must run in O(n) time.

#13 · Two Pointers

Valid Palindrome

After lowercasing and removing every non-alphanumeric character, a phrase is a palindrome if it reads the same forwards and backwards. Given a string, return true if it is a palindrome.

#14 · Two Pointers

3Sum

Given an integer array, return all unique triplets [a, b, c] with a + b + c = 0. The solution set must not contain duplicate triplets.

#15 · Two Pointers

Container With Most Water

Given n vertical lines of given heights, pick two of them that together with the x-axis form a container holding the most water, and return that amount. (Area = distance between the lines × the shorter height.)

#16 · Sliding Window

Minimum Window Substring

Given strings s and t, return the shortest substring of s that contains every character of t (including duplicates). Return an empty string if no such window exists.

#17 · Sliding Window

Longest Repeating Character Replacement

Given a string of uppercase letters and an integer k, you may replace at most k characters with any letter. Return the length of the longest substring that can be turned into a single repeated letter.

#18 · Linked List

Reverse Linked List

Given the head of a singly linked list, reverse the list and return the new head.

#19 · Linked List

Merge Two Sorted Lists

Merge two sorted linked lists into one sorted list by splicing together the nodes of the two lists, and return its head.

#20 · Linked List

Reorder List

Rearrange L0 → L1 → … → Ln−1 → Ln into L0 → Ln → L1 → Ln−1 → L2 → … in place, without changing any node values.

#21 · Linked List

Remove Nth Node From End of List

Given the head of a linked list, remove the n-th node from the end of the list and return its head.

#22 · Linked List

Linked List Cycle

Given the head of a linked list, return true if the list contains a cycle: some node can be reached again by continuously following next.

#23 · Linked List

Merge k Sorted Lists

Given an array of k sorted linked lists, merge them into one sorted linked list and return its head.

#24 · Stack

Valid Parentheses

Given a string containing only '(', ')', '{', '}', '[' and ']', determine whether it is valid: every bracket is closed by the same type, in the correct order.

#25 · Binary Search

Find Minimum in Rotated Sorted Array

A sorted array of unique values was rotated an unknown number of times (for example [0,1,2,4,5,6,7] became [4,5,6,7,0,1,2]). Return its minimum element in O(log n) time.

#26 · Binary Search

Search in Rotated Sorted Array

A sorted array of distinct values was rotated at an unknown pivot. Given a target, return its index, or −1 if it is absent, in O(log n) time.

#27 · Heap / Priority Queue

Top K Frequent Elements

Given an integer array and an integer k, return the k most frequent elements, in any order. The algorithm should be better than O(n log n).

#28 · Heap / Priority Queue

Find Median from Data Stream

Design a data structure that supports addNum(num), which inserts an integer from a data stream, and findMedian(), which returns the median of everything inserted so far.

#29 · Trees

Maximum Depth of Binary Tree

Given the root of a binary tree, return its maximum depth: the number of nodes on the longest path from the root down to the farthest leaf.

#30 · Trees

Same Tree

Given the roots of two binary trees p and q, return true if they are structurally identical and every pair of corresponding nodes has the same value.

#31 · Trees

Subtree of Another Tree

Given the roots of two binary trees root and subRoot, return true if some node of root together with ALL of its descendants is identical to subRoot.

#32 · Trees

Lowest Common Ancestor of a BST

Given a binary SEARCH tree and two of its nodes p and q, find their lowest common ancestor: the deepest node that has both p and q as descendants (a node counts as a descendant of itself).

#33 · Trees

Binary Tree Level Order Traversal

Given the root of a binary tree, return the values of its nodes level by level, from left to right, as a list of lists.

#34 · Trees

Validate Binary Search Tree

Given the root of a binary tree, decide whether it is a valid binary search tree: for EVERY node, all values in its left subtree are smaller and all values in its right subtree are larger.

#35 · Trees

Kth Smallest Element in a BST

Given the root of a binary search tree and an integer k, return the k-th smallest value (1-indexed) among all node values.

#36 · Trees

Construct Binary Tree from Preorder and Inorder Traversal

Given the pre-order and in-order traversals of a binary tree with distinct values, rebuild and return the tree.

#37 · Trees

Binary Tree Maximum Path Sum

A path is a sequence of nodes where each adjacent pair is connected by an edge; it need not pass through the root. Return the maximum sum of node values over all non-empty paths in the tree.

#38 · Trees

Serialize and Deserialize Binary Tree

Design an algorithm to turn a binary tree into a string (serialize) and to rebuild exactly the same tree from that string (deserialize). The encoding format is up to you.