subseq.c

#include <stdio.h>
#include <stdlib.h>

// num is the original number
// i is the current digit
// digits is the total number of digits in num
// mask is a number with 'digits' length of 1 and 0
void solve(int num, int i, int digits, int mask, int maxOne) {
	// combine mask and num and print only the digits of num
	// in the places where mask has 1
	if (i == digits) {
		while (digits--) {
			if (mask / maxOne)
				printf("%d", num / maxOne);
			num %= maxOne;
			mask %= maxOne;
			maxOne /= 10;
		}
		printf("  ");
		return;
	}
	// Either add another 1 in the mask
	solve(num, i+1, digits, mask * 10 + 1, maxOne);
	// or a 0
	solve(num, i+1, digits, mask * 10, maxOne);
}

int main(void) {
	int NUM = 28408;
	int t = NUM, digits = 1, maxOne = 1;
	// calculate num of digits
	// maxOne == 10^(digits-1)
	while (t > 9) {
		digits++;
		maxOne *= 10;
		t /= 10;
	}

	solve(NUM, 0, digits, 0, maxOne);
	printf("\n");
	
	return EXIT_SUCCESS;
}