题解 | #【模板】栈#

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Scanner;

public class Main {
    private static final String PUSH = "push";
    private static final String POP = "pop";
    private static final String TOP = "top";

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        final int NUM_OF_OP = in.nextInt();

        Deque<Integer> stack = new ArrayDeque<Integer>();

        for (int i = 0; i != NUM_OF_OP; ++i) {
            final String OP = in.next();
            if (OP.equals(PUSH)) {
                stack.push(in.nextInt());
            } else if (stack.peek() == null) {
                System.out.println("error");
            } else if (OP.equals(POP)) {
                System.out.println(stack.pop());
            } else {
                System.out.println(stack.peek());
            }
        }
    }
}