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.

class Solution {
public:
    string frequencySort(string s) {
        map<char, int> m;
        for (int i = 0;i < s.size();i++)
        {
            m[s[i]]++;
        }
        map<char, int>::iterator ite;
        set<int> vet;
        for (ite = m.begin();ite != m.end();ite++)
        {
            vet.insert(ite->second);
        }
        set<int>::iterator ite1;
        vector<int> vet1;
        for (ite1 = vet.begin();ite1 != vet.end();ite1++)
        {
            vet1.push_back(*ite1);
        }
        sort(vet1.begin(), vet1.end());
        string str = "";
        for (int i = vet1.size() - 1;i >= 0;i--)
        {
            for (ite = m.begin();ite != m.end();ite++)
            {
                if (ite->second == vet1[i])
                {
                    for (int j = 0;j < vet1[i];j++)
                    {
                        str += ite->first;
                    }
                }
            }
        }
        return str;
    }
};

#include<iostream>
#include<vector>
#include<string>
#include<map>
#include<algorithm>
#include<set>
using namespace std;
class Solution {
    public:
        string frequencySort(string s) {
                vector<vector<int>> pairs(256, vector<int>(2,1));
                for (int i = 0; i < pairs.size(); i++) {
                    pairs[i][1] = i;
                }
                for (char c : s)
                    pairs[c][0]++;
                string ret;
                sort(pairs.begin(), pairs.end());
                
                for (int i = (int)pairs.size() - 1; i >= 0; i--) {
                    for (int j = 0; j < pairs[i][0]; j++)
                        ret += (char)pairs[i][1];
                }
                return ret;
            }
        };
int main()
{
    Solution s;
    string str = "tree";
    cout << s.frequencySort(str) << endl;
    system("pause");
    return 0;
}