请不要直接 #include<stack>，题目要求实现一个栈！

#include <iostream>
#include <string>
using namespace std;
class stack {
  private:
    int s[100000];
    int top_index = -1;
  public:
    void push(int x) {
        top_index += 1;
        s[top_index] = x;
    }
    void pop() {
        if (top_index >= 0) {
            cout << s[top_index] << endl;
            top_index -= 1;
        } else
            cout << "error" << endl;
    }
    void top() {
        if (top_index >= 0)
            cout << s[top_index] << endl;
        else
            cout << "error" << endl;
    }
};
int main() {
    stack s;
    int n;
    cin >> n;
    for (int i = 0; i < n; i++) {
        string op;
        cin >> op;
        if (op == "push") {
            int x;
            cin >> x;
            s.push(x);
        }
        if (op == "pop") s.pop();
        if (op == "top") s.top();
    }
    return 0;
}