Leetcode - 49. Group Anagrams

Medium

Dec 22, 2025

#hashmap

#frequency

49. Group Anagrams

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

Example 1:

Input: strs = ["eat","tea","tan","ate","nat","bat"]

Output: [["bat"],["nat","tan"],["ate","eat","tea"]]

Explanation:

There is no string in strs that can be rearranged to form "bat".
The strings "nat" and "tan" are anagrams as they can be rearranged to form each other.
The strings "ate", "eat", and "tea" are anagrams as they can be rearranged to form each other.

Example 2:

Input: strs = [""]

Output: [[""]]

Example 3:

Input: strs = ["a"]

Output: [["a"]]

1 <= strs.length <= 104
0 <= strs[i].length <= 100
strs[i] consists of lowercase English letters.

Constraints:

Notes

Intuition

Anagrams have identical character frequencies. By using the frequency distribution as a key, all anagrams will hash to the same bucket.

Implementation

Use a hashmap where keys represent frequency distributions and values are lists of anagrams. For each string, build a 26-element frequency array. Use this as the key to group strings together. Finally, collect all the values from the map as the result.

Edge-cases

Python doesn't allow lists as dictionary keys. Convert the frequency array to a tuple or comma-separated string to make it hashable.

Time: O(n * k) — where n is the number of strings and k is the maximum string length
Space: O(n * k) — storing all strings in the hashmap

Complexity

Solution

from typing import List
class Solution:
    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
        map = {}
        for s in strs:
            freq = [0] * 26
            for c in s:
                freq[ord(c) - ord('a')] += 1

            freq_str = ",".join([str(f) for f in freq])
            if freq_str in map:
                map[freq_str].append(s)
            else:
                map[freq_str] = [s]

        ans = []
        for key in map:
            ans.append(map[key])

        return ans
        



if __name__ == "__main__":
    # Include one-off tests here or debugging logic that can be run by running this file
    # e.g. print(solution.two_sum([1, 2, 3, 4], 3))
    solution = Solution()
    print(solution.groupAnagrams(["eat", "tea", "tan", "ate", "nat", "bat"]))