Group Anagrams

Problem

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

Examples

The brute-force approach

For each pair of strings, check if they're anagrams by sorting both and comparing. Track which strings have already been grouped so you don't double-count.

visited = [False] * len(strs)
groups = []

for i in range(len(strs)):
    if visited[i]:
        continue
    group = [strs[i]]
    visited[i] = True
    for j in range(i + 1, len(strs)):   # every pair
        if sorted(strs[i]) == sorted(strs[j]):
            group.append(strs[j])
            visited[j] = True
    groups.append(group)

return groups

You're comparing every pair of strings, and sorting each one for every comparison. For n strings of average length k, that's O(n² * k log k). There's a way to group all anagrams in a single pass.

Spotting the pattern

This is a Hash Map problem. The key question to ask yourself:

What do all anagrams of a word have in common that I could use to group them?

Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.