USACO-Zero Sum

/*
ID:ay27272
PROG:zerosum
LANG:C++
*/

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <algorithm>

using namespace std;

int n,sum;
string result[100];
string f;

int number(int h,int &i)   //返回一个数字，包括空格的处理，当然当前处理到的位置也要返回，所以用了一个i的引用
{
int x=0;
i=h;
while (i<f.length() && f[i]!='+' && f[i]!='-')
{
if (f[i]!=' ')
x=x*10+int(f[i]-'0');
i++;
}
return x;
}

bool calc() //直接按照运算符号进行运算
{
int i,x=number(0,i);
int temp,k;
while (i<f.length())
{
k=i;
temp=number(i+1,i);
if (f[k]=='+') x+=temp;
else if (f[k]=='-') x-=temp;
}
return !x;
}

void dfs(int x)
{
if (x>n)
{
if (calc())
result[++sum]=f;
return;
}
string tmp=f;

f=f+' '+char(x+'0');
dfs(x+1);
f=tmp;

f=f+'+'+char(x+'0');
dfs(x+1);
f=tmp;

f=f+'-'+char(x+'0');
dfs(x+1);
f=tmp;
}

int main()
{
freopen("zerosum.in","r",stdin);
freopen("zerosum.out","w",stdout);
cin>>n;
sum=0;
f="1";
dfs(2);

for (int i=1;i<=sum;i++)
cout<<result[i]<<endl;

return 0;
}