栈 经典四则运算改款

#include <iostream>
#include <vector>
#include <string>
#include <utility>
#include <stack>

using namespace std;

int calculate(stack<pair<int, int>> &pstack) {
    int z = pstack.top().second;
    pstack.pop();
    int y = pstack.top().second;
    int x = pstack.top().first;
    pstack.pop();
    pstack.push({x, z});
    return x * y * z;
}

int main() {
    int n;
    int a, b;
    vector<pair<int, int>> mvec;
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> a >> b;
        mvec.push_back({a, b});
    }
    int result = 0;
    string str;
    cin >> str;
    stack<pair<int, int>> pstack;
    stack<char> ostack;
    for (int i = 0; i < str.size(); i++) {
        if (str[i] == '(') {
            ostack.push(str[i]);
        }
        else if (str[i] == ')') {
            result += calculate(pstack);
            ostack.pop();
        }
        else {
            int l = mvec[str[i] - 'A'].first;
            int r = mvec[str[i] - 'A'].second;
            pstack.push({l, r});
        }
    }
    cout << result << endl;
    
    return 0;
}