POJ 1426 Find The Multiple dfs

输出结果有点不能理解

#include <algorithm>#include <iostream>#include <stdio.h>#include <queue>#include <string.h>using namespace std;int n;int flag;void dfs(unsigned long long a, int k)//unsigned long long的最大值：18446744073709551615{    if(flag||k==19)    return ;    if(a%n==0){        printf("%I64u\n", a);        flag = 1;        return;    }else {        dfs(a*10,k+1);        dfs(a*10+1,k+1);    }}int main(){    while(~scanf("%d", &n)){        if(n == 0)  break;        flag = 0;        dfs(1,0);    }    return 0;}