矩阵连乘 LRJ白书 p141 栈 解析表达式

#include<iostream>
#include<cstdio>
#include<cstring>
#include<sstream>
#include<algorithm>
#include<queue>
#include<vector>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<fstream>
#include<memory>
#include<string>
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
#define MAXN  102
#define INF 1000000009
/*
利用栈解析表达式
*/
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;
    scanf("%d", &n);
    string str;
    for (int i = 0; i < n; i++)
    {
        cin >> str;
        int k = str[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)
            cout << "error\n";
        else
            cout << ans << endl;
    }
}