leetcode49 - Group Anagrams - medium

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.

 

用map来group，最直接的办法key是一个sorted string，假设L是最长的string长度，n个string，time complexity O(nLlogL)

优化：已知都是小写字母，可以用counting sort来序列化，拼接每个字母的freq作为key，couting花的时间好于sorting，最后time complexity O(nL)

 

实现：

class Solution {
private:
    string signature(string str){
        int counter[26] = {};
        for (char c : str){
            counter[c-'a']++;
        }
        string res;
        for (int i=0; i<26; i++){
            res += to_string(counter[i]) + '#';
        }
        return res;
    }
    
public:
    vector<vector<string>> groupAnagrams(vector<string>& strs) {
        
        vector<vector<string>> res;
        
        unordered_map<string, vector<string>> m;
        for (string s : strs){
            string sig = signature(s);
            m[sig].push_back(s);
        }

        for (auto row : m){
            res.push_back(row.second);
        }
        
        return res;
        
    }
};

 

注意init counter array的时候赋个起始值