使用栈计算

#include <iostream>
#include <vector>
#include <stack>

using namespace std;

pair<int,int> matrix_mul(pair<int,int> a,pair<int,int> b,int& sum) {
    sum += a.first * b.second * a.second;
    
    return pair<int,int>(a.first,b.second);
}

int main() {
    
    int num;
    cin >> num;
    
    vector<pair<int,int>> matrixs;
    
    int count = num;
    while(count--) {
        int a,b;
        cin >> a >> b;
        matrixs.push_back(pair<int,int>(a,b));
    }
    
    string s;
    cin >> s;
    
    string::iterator it_start = s.begin();
    string::iterator it_end = s.end();
    
//     stack<char> s;
    stack<pair<int,int>> sp;
    
    count = 0;
    int sum=0;
    while(it_start!= it_end) {
        
        switch(*it_start) {
            case '(': 
                break;
            case ')': {
                pair<int,int> b(sp.top());
                sp.pop();
                pair<int,int> a(sp.top());
                sp.pop();
                sp.push(matrix_mul(a,b,sum));
                break;
            }
            default: 
                sp.push(matrixs[count]);
                count++;
                break;
        }
        it_start++;
    }
    
    cout << sum;
    
    return 0;
}