NYOJ 267(郁闷的Ｃ小加(二)) 后缀表达式求值

悲剧了好几次，应该先判断栈是否为空，才能取栈顶元素，应该记住啊！

#include <iostream>
#include <stack>
#include<string>
#include<cstdlib>
#include<cstdio>
#include<algorithm>
using namespace std;
stack<double> num;
stack<char> sc;
double exp(double n1, double n2, char ch)
{
    switch(ch){
        case '+':return n1+n2;
        case '-':return n2-n1;
        case '*':return n1*n2;
        case '/':return n2/n1;
        default: break;
    }
}

bool isnum(char c)
{
    if((c >= '0' && c <= '9') || c == '.')
        return true;
    return false;
}

bool pre(char c1, char c2)
{
    if(c1 == ')' || c1 == '=')
        return false;
    else if(c2 == '(')
        return true;
    else if(c1 == '+' || c1 == '-')
        return false;
    else if((c1 == '*' || c1 == '/') && (c2 == '*' || c2 == '/'))
        return false;
    return true;
}

void out()
{
    double n1,n2;
    n1 = num.top(); num.pop();
    n2 = num.top(); num.pop();
    cout<<sc.top();
    num.push(exp(n1,n2,sc.top()));
    sc.pop();
}
void slove(string s)
{
    int i,j; char a[19];
    while(!sc.empty()) sc.pop();
    while(!num.empty()) num.pop();
    for(i=0; i<s.length(); ++i)
    {
        if(isnum(s[i]))
        {
            j = 0;
            while(isnum(s[i]))    a[j++] = s[i++];
            a[j] = '\0';  cout<<a;
            num.push(atof(a));  --i;
        }
        else
        {
            if(sc.empty() || pre(s[i],sc.top()))    sc.push(s[i]);
            else{
                if(s[i] == ')')
                {
                    while(sc.top() != '(')  out();
                    sc.pop();
                }
                else if(s[i] == '=')
                {
                    while(!sc.empty())    out();
                }
                else
                {
                    while(!sc.empty() && !pre(s[i],sc.top()) )    out();
                    sc.push(s[i]);
                }
            }
        }
    }
    cout<<'='<<endl;
    printf("%.2f\n",num.top());
    num.pop();
}

int main()
{
    int t;
    cin>>t;
    while(t--)
    {
        string s;
        cin>>s;
        slove(s);
        if(t>=1)
            cout<<endl;
    }
    return 0;
}