How Many Equations Can You Find

题目来源2017HPUACM暑期培训：https://vjudge.net/contest/174968#problem/E

 Now give you an string which only contains 0, 1 ,2 ,3 ,4 ,5 ,6 ,7 ,8 ,9.You are asked to add the sign ‘+’ or ’-’ between the characters. Just like give you a string “12345”, you can work out a string “123+4-5”. Now give you an integer N, please tell me how many ways can you find to make the result of the string equal to N .You can only choose at most one sign between two adjacent characters. 

Input
Each case contains a string s and a number N . You may be sure the length of the string will not exceed 12 and the absolute value of N will not exceed 999999999999.
Output
The output contains one line for each data set : the number of ways you can find to make the equation.
Sample Input

123456789 321 1

Sample Output

181

dfs的运用

#include<cstdio>#include<cmath>#include<cstring>#include<cstdlib>#include<algorithm>#include<iostream>#include<queue>#define ll long longusing namespace std;ll n,ans;int len;char s[15];void dfs(ll sum,ll x)//s时当前的和，x时现在的字符位置 {    if(x==len)    {        if(sum==n)        ans++;        return;    }    ll a=0;    for(int i=x;i<len;i++)    {        a=a*10+s[i]-'0';        dfs(sum+a,i+1);//接下来的几行时判断+ -号哪个符合         if(x!=0)        {            dfs(sum-a,i+1);        }    }}int main(){    while(~scanf("%s %lld",s,&n))    {        len=strlen(s);        ans=0;        dfs(0,0);        printf("%lld\n",ans);    }return 0;}