Find the two repeating elements in a given array.

repeated_numbers_countsort.cpp

// Problem link: https://www.geeksforgeeks.org/find-the-two-repeating-elements-in-a-given-array/

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

#define ull uint64_t

void countsort_rec(const vector<int> &ar, vector<int> &cnt, ull idx = 0) {
    if(idx == ar.size()) return;
    cnt[ar[idx]]++;
    countsort_rec(ar, cnt, idx+1);
}

void check_rec(const vector<int> &cnt, ull idx = 1) {
    if(idx == cnt.size()) 
        return;
    
    if(cnt[idx] == 2)
        cout << idx << " ";
    
    check_rec(cnt, idx+1);
}

void repeated_nums(vector<int> &ar) {
    vector<int> count(ar.size()+1, 0);
    countsort_rec(ar, count);
    check_rec(count);
}

int main() {
    int t;  cin >> t;
    while(t--) {
        int n;  cin >> n;
        
        vector<int> ar(n);
        for(int i = 0; i < n; ++i)
            cin >> ar[i];
        
        repeated_nums(ar);
        cout << endl;
    }
    return 0;
}