用递归！！

#include <stdio.h>

void change10_2(int n)
{
    if (n == 0)
        return;
    else
    {
        change10_2(n / 2);
        printf("%d", n % 2);
    }
}

int main(int argc, const char * argv[]) {
    int n;
    scanf("%d",&n);
    change10_2(n);
    return 0;
}

如果用python

#将10进制转为x进制
def change10_x(n, x):
    string = ''
    while n != 0:
        m = n % x
        string += str(m)
        n = int(n / x)
    return string[::-1]