Two Sum

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Problem

Given an array of integers nums and an integer target, return the indices of the two numbers that add up to target. Each input has exactly one valid answer, and you may not use the same element twice.

Examples

Example 1

Input:nums = [2,7,11,15], target = 9

Output:[0,1]

nums[0] + nums[1] = 2 + 7 = 9.

Example 2

Input:nums = [3,2,4], target = 6

Output:[1,2]

Constraints

2 <= nums.length <= 10^4
Only one valid answer exists for each input.

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How to approach it: the Arrays & Hashing pattern

Arrays give O(1) index access; hash maps and sets give O(1) average lookup, insert, and delete by trading space for speed. Together they're the toolkit for turning brute-force O(n²) comparisons into a single O(n) pass.

Look for this pattern when

You need to check "have I seen this value before?" quickly.
You're counting occurrences or grouping items by a derived key (e.g. sorted letters for anagrams).
A nested loop is comparing every pair — a hash map can often replace the inner loop.

Read the full Arrays & Hashing guide →

Video walkthroughs

Original problem on LeetCode ↗