49. Group Anagrams

LeetCode problem

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

An Anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once.

Example 1:

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

Example 2:

Input: strs = [""]
Output: [[""]]

Example 3:

Input: strs = ["a"]
Output: [["a"]]

Idea:

class Solution:
    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
        dd = {}
        for s in strs:
            s_sort = "".join(sorted(s))
            values = dd.get(s_sort, [])
            values.append(s)
            dd[s_sort] = values
        return dd.values()

Approach 2:

Intuition:

Two strings are anagrams if and only if their character counts (respective number of occurrences of each character) are the same.

Algorithm:

We can transform each string s into a character count, count\text{count}count, consisting of 26 non-negative integers representing the number of a’s, b’s, z’s, etc. We use these counts as the basis for our hash map.

In python, the representation will be a tuple of the counts. For example, abbccc will be (1, 2, 3, 0, 0, ..., 0), where again there are 26 entries total.

class Solution:
    def groupAnagrams(strs):
        res = collections.defaultdict(list)
        for s in strs:
            count = [0] * 26
            for c in s:
                count[ord(c) - ord('a')] += 1
            res[tuple(count)].append(s)
        return res.values()

Resources

• LeetCode expl