#Leetcode# 451. Sort Characters By Frequency

https://leetcode.com/problems/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.

代码：

class Solution {
public:
    string frequencySort(string s) {
        string ans = "";
        int len = s.length();
        unordered_map<char, int> mp;
        priority_queue<pair<int, char> > q;

        for(int i = 0; i < len; i ++)
            mp[s[i]] ++;

        for(auto i : mp)
            q.push({i.second, i.first});

        while(!q.empty()) {
            int k = q.top().first;
            char c = q.top().second;
            for(int i = 0; i < k; i ++)
                ans += c;
            q.pop();
        }

        return ans;
    }
};

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