How Many Equations Can You Find（DFS）

How Many Equations Can You Find

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 3
21 1
Sample Output
18
1

这道题在每位之间有三个分支（选择）：加号，减号，不作处理，

#include<iostream>
#include<algorithm>
#include<string>
using namespace std;

string s;
int a[21];
int n;
int ans;

void dfs(int k,int sum){
	if(k==s.size()){
		if(sum==n){//符合条件，进行回溯
		ans++;
		}	
		return;
	}
	for(int i=k;i<=s.size();i++){
		int ss=0;
		for(int j=k;j<=i;j++)
			ss=ss*10+s[j]-'0';//这位后面的数字均不作处理
			dfs(i+1,sum+ss);
			
			if(k>0) dfs(i+1,sum-ss);
		
	}
}

int main(){
	while(cin>>s>>n){
		ans=0;
		dfs(0,0);
		cout<<ans<<endl;
	}
	return 0;
}