解題思路：
十進位制轉二進位制方法，對2求餘，最後將餘數逆序輸出

#include<stdio.h>

int main()
{
    int num,a[20],i=0;
    printf("Input a number:");
    scanf("%d",&num);
    while(num < 0)
        exit(0);
    while(num != 0)
    {
        a[i] = num % 2;
        num = num / 2;
        i++;
    }
    i--;
    while(i >= 0
 
)
    {
        printf("%d",a[i]);
        i--;
    }
    return 0;

}