#include<bits/stdc++.h> using namespace std; int main(){ stack<int> mystack;//初始化栈 int n,num; string str1; while(cin >> n){ while(n--){ cin >> str1; if(str1 == "P"){ cin >> num; mystack.push(num); }else if(str1 == "O" && !mystack.empty()){ mystack.pop(); }else if(str1 =...