土法煉鋼，用 bitwise operation 應該會快些。
A basic method is used here, though bitwise operations should be faster.
ZeroJudge Link (Zh)
#include <stdio.h>
main() {
int num;
int buf[100];
int top;
while (scanf("%d", &num) != EOF) {
top = 0;
do {
buf[top++] = num % 2;
num /= 2;
} while (num != 0);
for (; top > 0; top--) printf("%d", buf[top-1]);
printf("\n");
}
return 0;
}