求助！

rt，只有16分。#2,#4RE，#3,#5,#6WA

#include<bits/stdc++.h>
using namespace std;
int main()
{
    string a;
    int i=0,s1,s2,ans=0;
    stack<int> num; 
    cin>>a;
    while(1)
    {
        if(a[i]=='@')
        {
            cout<<num.top();
            return 0;
        }
        if(a[i]<58 && a[i]>47)
            num.push(int(a[i]-48));
        if(a[i]=='+')
        {
            s1=num.top();
            num.pop();
            s2=num.top();
            num.pop();
            ans=s1+s2;
            num.push(ans);
        }
        if(a[i]=='-')
        {
            s1=num.top();
            num.pop();
            s2=num.top();
            num.pop();
            ans=s2-s1;
            num.push(ans);
        }
        if(a[i]=='*')
        {
            s1=num.top();
            num.pop();
            s2=num.top();
            num.pop();
            ans=s2*s1;
            num.push(ans);
        }
        if(a[i]=='/')
        {
            s1=num.top();
            num.pop();
            s2=num.top();
            num.pop();
            ans=s2/s1;
            num.push(ans);
        }
        i++;
    }
    return 0;
}

测本地没有问题啊！