Leetcode Sort Characters By Frequency

题意：按字符出现频率降序输出字符。

思路：用hash实现O(1)插入和查找。

class Solution {
public:
    string frequencySort(string s) {
       map<char, int> m;
       for(int i = 0; i < s.length(); ++ i) {
           m[s[i]] ++;
       }
       
       string re;
       int mmax = -1;
       while(mmax) {
           mmax = 0;
           std::map<char, int>::iterator it;
           std::map<char, int>::iterator mm;
           for(it = m.begin(); it != m.end(); ++ it) {
               if(mmax < it->second) {
                   mm = it;
                   mmax = it->second;
               }
           }
           if(mmax == 0) break;
           while(mm->second) {
               re += mm->first;
               mm->second --;
           }
       }
       //cout << re << endl;
       return re;
    }
};