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.

按照字符出现次数对字符串排序

C++:

class Solution {
public:
    string frequencySort(string s) {
        unordered_map<char,int> cntMap ;
        for(char c : s){
            cntMap[c]++ ;
        }
        vector<vector<char>> buckets(s.size()+1) ;
        for(auto m : cntMap){
            buckets[m.second].push_back(m.first) ;
        }
        string res = "" ;
        for(int i = buckets.size()-1 ; i >= 0 ; i--){
            for(char c : buckets[i]){
                for(int j = 0 ; j < i ; j++){
                    res += c ;
                }
            }
        }
        return res ;
    }
};