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20. Group Anagrams

mediumAsked at Notion

Bucket strings by their sorted-character signature — the same hash-map grouping Notion uses to cluster tags, block types, and database property values without scanning every entry on every lookup.

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

Problem

Given an array of strings, group the anagrams together and return each group as a list. The order of groups and words within groups does not matter.

Constraints

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

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; group words by key in a hash map.

Time
O(n * k log k) where k = max string length
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. Character frequency array as key

Build a 26-element frequency count per word; serialize it as a string key — avoids the k log k sort cost, useful when strings 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);
    for (const c of s) count[c.charCodeAt(0) - 97]++;
    const key = count.join(',');
    if (!map.has(key)) map.set(key, []);
    map.get(key).push(s);
  }
  return [...map.values()];
}

Tradeoff:

Notion-specific tips

Notion's block search and database grouping both hinge on fast canonical-key construction. They want you to compare the sort-based key vs. frequency-array key approaches and justify the choice based on string length. Also note that frequency arrays generalize to Unicode — mention it if time allows.

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Output

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