Group Anagrams

Problem

Given an array of strings, group anagrams together. Return the groups in any order.

Constraints

• 1 ≤ strs.length ≤ 10⁴
• 0 ≤ strs[i].length ≤ 100
• lowercase English

Approach — Hashing & Counting

This is a Hashing & Counting problem. The idea: store what you've seen in a hash map for O(1) lookups, trading a little space to avoid a nested scan. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: O(n) time, O(n) space.

Solution code

Python

class Solution:
    def groupAnagrams(self, strs):
        from collections import defaultdict
        d = defaultdict(list)
        for w in strs:
            d[''.join(sorted(w))].append(w)
        return sorted(sorted(g) for g in d.values())

Java

class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        Map<String, List<String>> groups = new HashMap<>();
        for (String s : strs) {
            char[] chars = s.toCharArray();
            Arrays.sort(chars);
            String key = new String(chars);
            groups.computeIfAbsent(key, k -> new ArrayList<>()).add(s);
        }
        return new ArrayList<>(groups.values());
    }
}

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.