UVa442 Matrix Chain Multiplication(栈)

#include<cstdio>
#include<cstring>
#include<stack>
#include<algorithm>
#include<iostream>
using namespace std;

struct matrix {
int a,b;
matrix(int a=0,int b=0):a(a),b(b) {}//结构体构造函数赋值
}m[26];

stack<matrix>s;

int main()
{
    int n;
    char A;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    {
        cin >> A;
        int k=A-‘A‘;
        cin >> m[k].a >> m[k].b;
    }
    string expr;
    while(cin >> expr)
    {
        int len = expr.length();
        int flag = 0;
        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)
                {
                    flag = 1;
                    break;
                }
                ans += m1.a*m1.b*m2.b;
                s.push(matrix(m1.a,m2.b));
            }
        }
        if(flag) printf("error\n");
        else printf("%d\n",ans);
    }
    return 0;
}