451. Sort Characters By Frequency

Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:"tree"Output:"eert"Explanation:'e' appears twice while 'r' and 't' both appear once.So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

Example 2:

Input:"cccaaa"Output:"cccaaa"Explanation:Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.Note that "cacaca" is incorrect, as the same characters must be together.

Example 3:

Input:"Aabb"Output:"bbAa"Explanation:"bbaA" is also a valid answer, but "Aabb" is incorrect.Note that 'A' and 'a' are treated as two different characters.

用 priority_queue:

class Solution {public:    struct charNum{        char c;        int num;        charNum(char a,int b){            c = a;num = b;        }        friend bool operator < (charNum a , charNum b){            return (a.num < b.num);        }    };        string frequencySort(string s) {        map<char,int> m;        for(char c:s){            m[c]++;        }        priority_queue<charNum> q;        for(auto& cn : m){            q.push(charNum(cn.first, cn.second));        }        string str;        while(!q.empty()){            auto i = q.top();            q.pop();            str.append(i.num,i.c);        }        return str;    }};