C :

#include <stdio.h>
int main ()
{
	//freopen("a.in","r",stdin);
	//freopen("a.out","w",stdout);
	int n, k, a[10000];
	while(scanf("%d",&n) != EOF)
	{
		k = 0;
		while(n != 0)
		{
			a[k++] = n % 2;
			n /= 2;
		}

		for(int i = k - 1 ; i >= 0 ; i --)
			printf("%d",a[i]);
		printf("\n");
	}
	return 0;
}

C++ :

#include <stdio.h>
void print2(int n)
{
	int i;
	i = n % 2;
	n = n / 2;
	if (n != 0)
		print2(n);
	printf("%d", i);
}

int main()
{
	int n;
	while (scanf("%d", &n) != EOF)
	{
		print2(n);
		printf("\n");
	}
	return 0;
}