LeetCode 49. Group Anagrams

题目描述:

Given an array of strings, group anagrams together.

For example, given: [“eat”, “tea”, “tan”, “ate”, “nat”, “bat”], Return:

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

anagram的意思是"颠倒字母而成的词句", 也就是要把由相同字母组成的但顺序不同的字符串放到一起. 使用hash表, 对每个字符串中的字符排序后得到的新字符串作为hash表的key, value则是对应的原字符串集合.

class Solution {
public:
    vector<vector<string>> groupAnagrams(vector<string>& strs) {
        unordered_map<string, vector<string>> m;
        vector<vector<string>> ret;
        for(int i = 0; i < strs.size(); i++){
            string s = strs[i];
            sort(s.begin(), s.end());
            if(m.count(s)){
                m[s].push_back(strs[i]);
            }
            else{
                m[s] = vector<string>(1, strs[i]);
            }
        }
        ret.resize(m.size()); //预先扩展ret的大小, 避免在循环中push_back频繁分配新的内存空间
        int cnt = 0;
        for(auto i = m.begin(); i != m.end(); i++){
            ret[cnt++] = (i->second);
        }
        return ret;
    }
};