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20. Group Anagrams

mediumAsked at Square

Cluster merchant names that are anagrams of each other so Square's compliance team can batch-review aliases — a hash-map keyed on sorted character signatures groups thousands of merchant strings in a single linear pass.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an array of strings strs, group the anagrams together. You can return the answer in any order.

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 string as key

Sort the characters of each word to produce a canonical key; use a map from key to list of words. Simple, works, but sorting each word is O(k log k).

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].sort().join('');
    if (!map.has(key)) map.set(key, []);
    map.get(key).push(s);
  }
  return [...map.values()];
}

Tradeoff:

2. Frequency count as key

Build a 26-integer count array for each word and stringify it as the map key. Avoids sorting — O(n * k) overall, better when words are long.

Time
O(n * k)
Space
O(n * k)
function groupAnagrams(strs) {
  const map = new Map();
  for (const s of strs) {
    const count = new Array(26).fill(0);
    const a = 'a'.charCodeAt(0);
    for (const c of s) count[c.charCodeAt(0) - a]++;
    const key = count.join(',');
    if (!map.has(key)) map.set(key, []);
    map.get(key).push(s);
  }
  return [...map.values()];
}

Tradeoff:

Square-specific tips

Square sees two variants: which grouping strategy you pick, and whether you can articulate the tradeoff between sort-key vs count-key under different input shapes. Mention that sort-key is simpler to read in a code review while count-key wins when average word length k is large. Readable code that ships wins over micro-optimized code that sits in review.

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Output

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