451. Sort Characters By Frequency(桶排序)

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.

class Solution {
public:
    //桶排序。将相同长度的子串放到同一个桶中，最后利用桶排序
    string frequencySort(string s) {
　　　　 //防止s="aaa"  插入 b[3] = "aaa";
        vector<string> bucket(s.size()+1,"");
        map<char,int> m;
        string res;
        for(int i=0;i<s.size();i++){
            m[s[i]]++;
        }
        for(auto sub:m){
            int n = sub.second;
            int c = sub.first;
            //长度为n的子串都放到bucket[n]
            bucket[n].append(string(n,c));
        }
        //最后利用桶排序，倒序遍历桶即可
        for(int i=bucket.size()-1;i>=0;i--){
            res.append(bucket[i]);
        }
        return res;
    }
};