题解 | 【模板】队列操作

import java.util.*;
public class Main{
    public static void main(String[] args){
        Queue<Integer> queue = new LinkedList<>();
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        while(n-->0){
            int i = in.nextInt();
            switch(i){
                case 1:
                queue.offer(in.nextInt());
                break;

                case 2:
                if(queue.size()!=0){
                    queue.poll();
                    break;
                }
                System.out.println("ERR_CANNOT_POP");
                break;

                case 3:
                if(!queue.isEmpty()){
                    System.out.println(queue.peek());
                    break;
                }
                System.out.println("ERR_CANNOT_QUERY");
                break;

                case 4:
                System.out.println(queue.size());
                break;
            }
        }
    }
}