矩阵链乘（UVa 442)

　　结构体 struct matrix 用来保存矩阵的行和列；

　　map<string,matrix> 用来保存矩阵名和相应的行列数；

　　stack<string> 用来保存表达式中遇到的矩阵名，并将每次乘法运算后的矩阵压入栈中；

C++11 代码如下：

#include<iostream>
#include<map>
#include<string>
#include<stack>
using namespace std;

struct matrix {
    int row;
    int column;
};

int main() {
    int n; string ch;
    string str;
    int li, co;
    map<string, matrix>m;
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> ch >> li >> co;
        m[ch].row = li;
        m[ch].column = co;
    }
    while ((cin >> str)){    
        stack<string>s;
        string a, b;
        int sum = 0;
        bool flag = true;
        for (int i = 0; i < str.length(); i++) {
            if (str[i] == '(') continue;
            else if (str[i] == ')') {
                a = s.top(); s.pop();
                b = s.top(); s.pop();
                if (m[b].column == m[a].row) {
                    sum += m[b].row*m[b].column*m[a].column;
                    string ba;
                    ba = b + a;
                    s.push(ba);
                    m[ba].row = m[b].row;
                    m[ba].column = m[a].column;
                }
                else {
                    flag = false;
                    break;
                }
            }
            else {
                string s2;
                s2 = str[i];
                s.push(s2);
            }
        }
        if(flag) cout << sum << endl;
        else cout << "error" << endl;
    }
    return 0;
}