【50】451. Sort Characters By Frequency

451. Sort Characters By Frequency

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• Total Accepted: 17072
• Total Submissions: 34068
• Difficulty: Medium
• Contributors: stickypens

 

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.

class Solution {
public:
    string frequencySort(string s) {
        string res = "";
        unordered_map<char, int> hash;//用来记录谁出现了几次
        priority_queue<pair<int, char>> q;//用来直接排序，把map的两个值反过来记录
        for(char c : s){
            hash[c]++;
        }
        for(auto a : hash){
            q.push(make_pair(a.second, a.first));
        }
        while(!q.empty()){
            auto top = q.top();
            q.pop();
            //    string& append (size_t n, char c);
            //Appends n consecutive copies of character c.
            res.append(top.first, top.second);//把谁append，加上几次
        }
        return res;
    }
};