#include <stdio.h>

#define N 4

void binstrs(char bits[], int index) {
    if (index == N) {
        printf("%s\n", bits);
        return;
    }

    bits[index] = '0';
    binstrs(bits, index + 1);

    bits[index] = '1';
    binstrs(bits, index + 1);
}

int main() {
    char binary_num[N + 1];
    binary_num[N] = '\0';

    binstrs(binary_num, 0);
    return 0;
}