Group Anagrams

Given an array of strings strs, group all anagrams together into sublists. You may return the output in any order.

An anagram is a string that contains the exact same characters as another string, but the order of the characters can be different.

Approach

Use a hash map from a 26-length character count signature to the list of strings with that signature. For each string, build a count array for lowercase letters, convert it to a tuple key, and append the string to that group. This cleanly handles empty strings and repeated characters.

Why It Works

Two strings are anagrams if and only if their character frequency counts are identical. The count tuple is therefore a canonical signature for all anagrams, so grouping by that key places exactly the correct strings together.

Complexity

• Time: O(n * k), where k is the max string length
• Space: O(n * k) for stored output and keys

Reference Implementation (Python)

class Solution:
    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
        groups = defaultdict(list)
        for s in strs:
            counts = [0] * 26
            for ch in s:
                counts[ord(ch) - ord('a')] += 1
            groups[tuple(counts)].append(s)
        return list(groups.values())