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15. Group Anagrams

mediumAsked at Revolut

Group strings that are anagrams of each other under a canonical key, a Revolut screen that mirrors bucketing different spellings of the same merchant name for ledger reconciliation.

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

Problem

Given an array of strings, group the anagrams together. Return the list of groups in any order; strings within a group may also be in any order.

Constraints

  • 1 <= strs.length <= 10^4
  • 0 <= strs[i].length <= 100
  • Lowercase English letters

Examples

Example 1

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

Example 2

Input
strs=[""]
Output
[[""]]

Approaches

1. Sorted-string key

Use sorted characters as the bucket key.

Time
O(n * k log k)
Space
O(n*k)
const m = new Map();
for (const s of strs){ const k = [...s].sort().join(''); if (!m.has(k)) m.set(k,[]); m.get(k).push(s); }
return [...m.values()];

Tradeoff:

2. Count-array key

Use a 26-length character-count vector as the key. Linear in total characters.

Time
O(n*k)
Space
O(n*k)
function groupAnagrams(strs){
  const m = new Map();
  for (const s of strs){
    const cnt = new Array(26).fill(0);
    for (const c of s) cnt[c.charCodeAt(0)-97]++;
    const k = cnt.join('#');
    if (!m.has(k)) m.set(k, []);
    m.get(k).push(s);
  }
  return [...m.values()];
}

Tradeoff:

Revolut-specific tips

Revolut frames this as merchant-name canonicalization for transaction enrichment — they expect you to mention case-folding and trimming as a real-world preprocessing step.

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Output

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