矩阵链乘UVa442

题目链接：

UVa442

代码：

#include <iostream>
#include <cstdio>
#include <stack>
#include <string>

using namespace std;

struct Matrix{
    int a , b;
    Matrix(int a=0,int b=0):a(a),b(b){}
}m[26];

stack<Matrix> s;

int main()
{   int n;
    cin>>n;
    for(int i = 0;i < n;i++){
        string name;
        cin >> name;
        int k = name[0] - 'A';
        cin >> m[k].a>>m[k].b;
    }
    string expr;
    while(cin >> expr){
        int len = expr.length();
        bool error = false;
        int ans = 0;
        for(int i = 0;i < len;i++){
            if(isalpha(expr[i]))s.push(m[expr[i] - 'A']);
            else if (expr[i] == ')'){
                Matrix m2 = s.top();s.pop();
                Matrix m1 = s.top();s.pop();
                if(m1.b != m2.a){error =true;break;}
                ans += m1.a*m1.b*m2.b;
                s.push(Matrix(m1.a,m2.b));
            }
        }
        if(error) printf("error\n");else printf("%d\n",ans);
    }
    //cout << "Hello world!" << endl;
    return 0;
}