UVA 442(p141)----Matrix Chain Multiplication

#include<iostream>
#include<cstdio>
#include<stack>
#include<cstring>
#include<algorithm>
using namespace std;
struct point
{
    int a,b;
    point(int x=0,int y=0):a(x),b(y) {}
};
int n;
point m[30];
stack<point> s;
int main()
{
    cin>>n;
    for(int i=0; i<n; i++)
    {
        char a;
        int x,y;
        cin>>a>>x>>y;
        int tmp=a-'A';
        m[tmp].a=x;
        m[tmp].b=y;
    }
    string st;
    while(cin>>st)
    {
        int l=st.length();
        int flag=0,ans=0;
        for(int i=0; i<l; i++)
        {
            if(isalpha(st[i]))
                s.push(m[st[i]-'A']);
            else if(st[i]==')')
            {
                point x=s.top();
                s.pop();
                point y=s.top();
                s.pop();
                if(y.b!=x.a)
                {
                    flag=1;
                    break;
                }
                else
                {
                    ans+=y.a*y.b*x.b;
                    s.push(point(y.a,x.b));
                }
            }
        }
        if(flag) cout<<"error"<<endl;
        else cout<<ans<<endl;
    }
    return 0;
}

题目地址：https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=6&page=show_problem&problem=383