题目描述：

Description

Given a positive integer n, write a program to find out a nonzero multiple m of n whose decimal representation contains only the digits 0 and 1. You may assume that n is not greater than 200 and there is a corresponding m containing no more than 100 decimal digits.
Input

The input file may contain multiple test cases. Each line contains a value of n (1 <= n <= 200). A line containing a zero terminates the input.
Output

For each value of n in the input print a line containing the corresponding value of m. The decimal representation of m must not contain more than 100 digits. If there are multiple solutions for a given value of n, any one of them is acceptable.
Sample Input

2
6
19
0
Sample Output

10
100100100100100100
111111111111111111

题目分析：

这题看到1、0很容易误以为是一道跟二进制有关的题，但实际上本题跟二进制没什么关系。题意是给你一个数字，让你在这个数的任意倍数中找到一个只有1和0组成的十进制的数字。看似很难，但只需要用到一个很简单的深度优先搜索。

代码：

#include <iostream>
#include<algorithm>
#define ll long long
using namespace std;
int flag;
ll n,ans;
void DFS(ll x,int step){
    if(step>18||flag){
        return;
    }
    if(x%n==0){
        ans=x;
        flag=1;
        return;
    }
    DFS(x*10,step+1);
    DFS(x*10+1,step+1);
}
int main() {
    while(cin>>n&&n){
        flag=0;
        DFS(1,0);
        cout<<ans<<endl;
    }
    return 0;
}