Sorting elements of an Array by Frequency aka Frequency Sort.

frequency_sort.cpp

// Problem link: https://practice.geeksforgeeks.org/problems/sorting-elements-of-an-array-by-frequency/0/

#include <vector>
#include <climits>
#include <iostream>
#include <algorithm>
using namespace std;

#define ull uint64_t

void frequency_sort(int n, int m = 1, int M = 60) {
    vector<vector<int>> count(M-m+1, vector<int>(2, 0));
    for(ull i = 0; i < count.size(); ++i)
        count[i][0] = i+m;
    
    int x;
    for(int i = 0; i < n; ++i) {
        cin >> x;
        count[x-m][1]++;
    }
    
    int max_freq = 0;
    vector<vector<int>> real_count;
    for(ull i = 0; i < count.size(); ++i) 
        if(count[i][1]) {
            vector<int> temp{count[i][0], count[i][1]};
            real_count.push_back(temp);
            max_freq = max(max_freq, count[i][1]);
        }
    
    
    for(int k = max_freq; k >= 1; --k)
        for(ull i = 0; i < real_count.size(); ++i)
            if(real_count[i][1] == k)
                for(int j = 0; j < k; ++j)
                    cout << real_count[i][0] << " ";
    
    cout << endl;
}

int main() {
    int t;  cin >> t;
    while(t--) {
        int n;  cin >> n;
        frequency_sort(n);
    }
    return 0;
}