49.Group Anagrams

Tags: Medium Hash Table String

Links: https://leetcode.com/problems/group-anagrams/

---

Given an array of strings, group anagrams together.

Example:

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

Note:

• All inputs will be in lowercase.
• The order of your output does not matter.

---

class Solution {
public:
    vector<vector<string>> groupAnagrams(vector<string>& strs) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        unordered_map<string, vector<int>> m;
        int n = strs.size();
        for (int i = 0; i < n; ++i){
            string tmp = strs[i];
            sort(tmp.begin(), tmp.end());
            m[tmp].push_back(i);
        }

        vector<vector<string>> res;
        for (auto & e : m) {
            vector<string> v;
            int len = e.second.size();
            for (int i = 0; i < len; ++i) {
                v.push_back(strs[e.second[i]]);
            }
            res.push_back(v);
        }

        return res;
    }
};

题意是把拥有相同字母的单词组合在一起输出，那么一个很直接的判断方法就是对单词进行排序，那么相同字母组合的排序后一定相等，题目又要求输出原单词，那么就每次将单词复制一份再排序。

考虑到要合并同类，那么很容易想到用字典，这里值的部分不需要保存单词，浪费空间，并且复制字符串耗时间，只需要记录这个单词在数组中的下标即可。