题解 | #【模板】队列#

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

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {

    private static Deque<Integer> queue = new ArrayDeque<>();
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        final int NUM = in.nextInt();

        for (int i = 0; i != NUM; ++i) {
            String str = in.next();
            if (str.equals("push")) {
                queue.addLast(in.nextInt());
            } else if (str.equals("pop")) {
                if (queue.isEmpty()) {
                    System.out.println("error");
                } else {
                    System.out.println(queue.pollFirst());
                }
            } else {
                if (queue.isEmpty()) {
                    System.out.println("error");
                } else {
                    System.out.println(queue.peekFirst());
                }                
            }
        }
    }
}