Group Anagrams

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Problem

Given an array of strings, group the strings that are anagrams of each other into separate lists, and return the list of groups in any order.

Examples

Example 1

Input:strs = ["eat","tea","tan","ate","nat","bat"]

Output:[["eat","tea","ate"],["tan","nat"],["bat"]]

Example 2

Input:strs = [""]

Output:[[""]]

Constraints

1 <= strs.length <= 10^4
strs[i] consists of lowercase English letters only.

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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 ↗