华为机试：矩阵乘法计算量估算

#include <iostream>
#include <vector>
#include <stack>
using namespace std;

int main() {
    int n;
    cin >> n;
    vector<pair<int, int>> C;
    for (int i = 0; i < n; i++) {
        int row, col;
        cin >> row >> col;
        C.push_back(make_pair(row, col));
    }
    stack<pair<int, int>> ch;
    stack<char> op;
    char c;
    int sum = 0;
    while (cin >> c) {
        if (c == '(') {
            op.push(c);
        } 
        else if (c >= 'A' && c <= 'Z') {
            ch.push(C[c - 'A']);
        } 
        else {
            auto i = ch.top();
            ch.pop();
            auto j = ch.top();
            ch.pop();
            sum += j.first * j.second * i.second;
            ch.push(make_pair(j.first, i.second));
            op.pop();
        }
    }
    cout << sum << endl;
    return 0;
}