Prints $digits-permutation of 1 to $digits recursively

recursive.c

#include <stdio.h>

int digits, level;
int number[20];
int used[10];

void print() {
    for (int i = 0; i < digits; i++)
        printf("%d", number[i]);
    printf("\n");
}

void gen() {
    if (level == digits) {
        print();
        return;
    }
    for (int i = 1; i <= digits; i++) {
        if (used[i]) continue;
        number[level] = i;
        used[i] = 1;
        level++;
        gen();
        level--;
        used[i] = 0;
    }
}

int main() {
    scanf("%d", &digits);
    gen();

    return 0;
}