矩陣乘法計算量估算（C++牛客網）

解題思路：

（1）兩個棧

（2）注意棧為空的情況

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

int helper(vector<vector<int>> &v,string &s) {
    stack<char> s1;
    stack<pair<int,int>> s2;
    int count=0,sum=0;
    for(int i=0;i<s.length();i++) {
        if(s[i]=='(') s1.push(s[i]);
        else if('A'<=s[i] && s[i]<='Z') {
            s2.push({v[count][0],v[count][1]});
            count++;
        } else {
            if(s2.empty()) return sum; 
            auto a=s2.top();
            s2.pop();
            if(s2.empty()) return sum; 
            auto b=s2.top();
            s2.pop();
            sum+=b.first*b.second*a.second;
            s2.push({b.first,a.second});
            s1.pop();
            
        }
    }
    return sum;
}

int main() {
    int n;
    string s;
    while(cin>>n) {
        vector<vector<int>> v(n,vector<int>(2,0));
        for(int i=0;i<n;i++) {
            cin>>v[i][0]>>v[i][1];
        }
        cin>>s;
        cout<<helper(v,s)<<endl;
    }
    return 0;
}