LeetCode 49: Group Anagrams — Python Solution

Solve LeetCode 49: Group Anagrams in Python with a canonical frequency key approach. The key is to make the state invariant explicit, so the implementation and complexity follow naturally.

This guide paraphrases the task and does not reproduce LeetCode’s prompt. Use the official page for the complete statement, examples, constraints, and submission runner.

What the problem is testing

Convert each word into a 26-count tuple and group words sharing that immutable signature.

Algorithm

1. Convert each word into a 26-count tuple and group words sharing that immutable signature.
2. Maintain this invariant: Words share a group exactly when their character-count signatures match.
3. Continue until every input item or reachable state has been resolved, then return the accumulated result.

Python solution

from collections import Counter, defaultdict, deque, OrderedDict
import random

class Solution:
    def groupAnagrams(self, strs):
        groups = defaultdict(list)
        for word in strs:
            counts = [0] * 26
            for char in word: counts[ord(char) - ord("a")] += 1
            groups[tuple(counts)].append(word)
        return list(groups.values())

Why this is correct

The proof follows the maintained state: Words share a group exactly when their character-count signatures match. Each iteration preserves that claim while permanently resolving at least one position, node, interval, or search state. When the loop or recursion ends, every candidate required by the problem has therefore been included or ruled out, so the returned value is correct.

Complexity

O(total characters) time and O(total characters) output space. The stated auxiliary space excludes the returned output unless the output is the data structure being built.

Edge cases

Empty strings share the all-zero signature; repeated words remain repeated outputs.

Tested reference code

This implementation is included in the site’s downloadable 100-solution Python library. The complete suite compiles every solution and runs a behavioral assertion for every problem before publication.

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