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Easy · Hash map

Two Sum

Classic hash-map scan. Name brute force, then the map, walk [2,7,11,15] target 9, finish with O(n).

Medium · Hash map

Group Anagrams

Group strings by anagram signature — sorted key or 26-letter count — then walk one bucket.

Easy · Hash map

Contains Duplicate

Scan once into a set; if insert fails, a duplicate exists.

Easy · Two pointers

Valid Palindrome

Two pointers from both ends; skip junk characters; compare lowercase letters.

Medium · Two pointers

3Sum

Sort, fix i, then two pointers for the pair that sums to −nums[i]. Deduplicate out loud.

Medium · Sliding window

Longest Substring Without Repeating Characters

Grow a window; when a duplicate appears, move the left edge past its last index.

Easy · Stack

Valid Parentheses

Stack story: push openers, pop on matching closer, empty stack at the end.

Medium · Stack

Daily Temperatures

Monotonic stack of indices waiting for a warmer day; answer[i] is days to wait.

Easy · Binary search

Binary Search

Sorted array: left, right, mid — discard the half that cannot contain the target.

Medium · Binary search

Search in Rotated Sorted Array

Rotated sorted array: at mid, one side is sorted — decide which half can hold the target.

Medium · Intervals

Merge Intervals

Sort by start; if the next start ≤ current end, extend the end; else push a new interval.

Easy · Linked list

Reverse Linked List

Iteratively reverse pointers with prev/curr/next — or recurse and narrate the unwind.

Easy · Linked list

Linked List Cycle

Slow moves one, fast moves two; if they meet there is a cycle.

Easy · Trees

Maximum Depth of Binary Tree

Depth is 1 + max(left, right). Narrate base case null → 0.

Medium · Trees

Binary Tree Level Order Traversal

BFS with a queue; for each level, process queue length nodes into one list.

Medium · Graphs

Number of Islands

Treat the grid as a graph; each unvisited land starts a DFS/BFS that sinks the island.

Easy · Dynamic programming

Climbing Stairs

Bottom-up Fibonacci: each step comes from one or two stairs below.

Medium · Dynamic programming

Coin Change

DP array dp[a] = min coins to make amount a; try each coin.

Medium · Design

LRU Cache

Map key → node, doubly linked list for recency; get/put both O(1).