26. Group Anagrams

mediumAsked at Etsy

Cluster strings that are anagrams of each other — Etsy uses this hash-key normalization pattern to deduplicate misspelled listing tags ('handmade', 'hnadmade') before indexing.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-26

Problem

Given an array of strings strs, group the anagrams together. You can return the answer in any order. An anagram is a word or phrase formed by rearranging the letters of another, using all the original letters exactly once.

Constraints

1 <= strs.length <= 10^4
0 <= strs[i].length <= 100
strs[i] consists of lowercase English letters

Examples

Example 1

Input

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

Output

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

Example 2

Input

strs = [""]

Output

[[""]]

Approaches

1. Sort each word as key

Sort the characters of each word alphabetically to produce a canonical key. Group words by key using a Map. Two anagrams produce the same sorted key. O(n * k log k) where k is the max word length.

Time: O(n * k log k)
Space: O(n * k)

function groupAnagrams(strs) {
  const map = new Map();
  for (const s of strs) {
    const key = s.split('').sort().join('');
    if (!map.has(key)) map.set(key, []);
    map.get(key).push(s);
  }
  return [...map.values()];
}

Tradeoff:

2. Frequency count as key

Instead of sorting, build a 26-length frequency array for each word and stringify it as the key (e.g., '1,0,0,...,1,...'). Avoids the sort's log factor — O(n * k) overall.

Time: O(n * k)
Space: O(n * k)

function groupAnagrams(strs) {
  const map = new Map();
  for (const s of strs) {
    const freq = new Array(26).fill(0);
    for (const ch of s) freq[ch.charCodeAt(0) - 97]++;
    const key = freq.join(',');
    if (!map.has(key)) map.set(key, []);
    map.get(key).push(s);
  }
  return [...map.values()];
}

Tradeoff:

Etsy-specific tips

Etsy uses anagram grouping for tag normalization and search synonym expansion. Interviewers appreciate when you articulate the key insight: 'two strings are anagrams if and only if their sorted forms are identical' — then pivot to the frequency approach to show you can eliminate the sort. Mention that the frequency key approach is more general and extends to Unicode if you replace the 26-slot array with a Map.

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