题解 | #堆栈的使用#

#include <iostream>
#include <cstdio>
#include <stack>

using namespace std;

/**
 * 堆栈的使用
 * @return
 */
int main() {
    int n;
    while (cin >> n) {
        stack<int> stack;
        for (int i = 0; i < n; ++i) {
            char instruction;
            cin >> instruction;
            if (instruction == 'P') {
                int num;
                cin >> num;
                stack.push(num);
            } else if (instruction == 'O') {
                if (!stack.empty()) {
                    stack.pop();
                }
            } else {
                if (stack.empty()) {
                    cout << "E" << endl;
                } else {
                    cout << stack.top() << endl;
                }
            }
        }
    }
    return 0;
}