C++ Programming code logic explanation

i have a question for recursion in C++,i read a lot of posts,but i still can't get the logic in this exam i have.If you please help me what is the exact logic and if it possible to become easier i would be gratefull. If i enter N=3 and R=2 the answer is 12,13,21,23,31,32. But what is the logic?

#include <iostream>
using namespace std;
#define MAXN 20


int n, r;

char used[MAXN] = { 0 };
int mp[MAXN];

void permute(int i);
void print();

int main()
{
    int i=0;
    /*cout << "enter r";
    cin >> r;
    cout << "enter n";
    cin >> n;*/
    cout << used[0];
    n = 3;
    r = 2;
    permute(i);

}

void permute(int i)
{
    int k;
    if (i >= r)
    {
        print();
        return;
    }

    for (k = 0; k<n; k++)
    {
        if (!used[k])
        {
            used[k] = 1;
            mp[i] = k;
            permute(i + 1);
            used[k] = 0;
        }
    }
}

void print()
{
    int i;
    for (i = 0; i<r; i++)
    {
        cout << mp[i] + 1;
    }
    cout << endl;
}