Day22. SortCharacterByFrequency.cpp

/*

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) {
        int count[256] = {0};
        for(char c : s)
            count[c] += 1;
        vector<pair<int,char>> arr;
        for(int c = 0; c < 256; ++c)
        {
            if(count[c]) arr.push_back({count[c],c});
        }
        sort(arr.rbegin(), arr.rend());
        string res;
        for(auto p : arr)
        {
            res += string(p.first, p.second);
        }
        return res;
    }
};