Combinations with repetitions using C++

#include <bits/stdc++.h>

using namespace std;

vector<vector<int>> res;

void CombinationRepetitionUtil(int chosen[], int arr[], int index, int r, int start, int end) {
    if (index == r) {
        vector<int> vec;
        for (int i = 0; i < r; i++) {
            vec.push_back(arr[chosen[i]]);
        }
        res.push_back(vec);
        return;
    }
    for (int i = start; i <= end; i++) {
        chosen[index] = i;
        CombinationRepetitionUtil(chosen, arr, index + 1, r, i, end);
    }
    return;
}

void CombinationRepetition(int arr[], int n, int r) {
    int chosen[r + 1];
    CombinationRepetitionUtil(chosen, arr, 0, r, 0, n - 1);
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int arr[] = {0, 1, 2, 3};
    int n = sizeof(arr) / sizeof(arr[0]);
    int r = 2;
    // generate all combinations of size r (using the elements from the array)
    CombinationRepetition(arr, n, r);
    for (int i = 0; i < (int) res.size(); i++) {
        for (int j = 0; j < (int) res[i].size(); j++) {
            cout << res[i][j] << " ";
        }
        cout << '\n';
    }
    return 0;
}