题解 | 【模板】队列操作

import java.util.ArrayDeque;
import java.util.Queue;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Queue<Integer> queue=new ArrayDeque<>();
		Scanner scanner=new Scanner(System.in);
		int n=scanner.nextInt();
		for (int i = 0; i < n; i++) {
			int a=scanner.nextInt();
			if(a==1) {
				int b=scanner.nextInt();
				queue.add(b);
			}else if(a==2) {
				if(queue.isEmpty()) {
					System.out.println("ERR_CANNOT_POP");
				}else {
					queue.poll();//队列的出队操作
				}
			}else if(a==3) {
				if(queue.isEmpty()) {
					System.out.println("ERR_CANNOT_QUERY");
				}else {
					System.out.println(queue.peek());
				}
			}else if(a==4) {
				System.out.println(queue.size());
			}
		}

	}

}

常见的队列方法，add():入队，poll():出队，peek()：查看队顶元素，size()查看队列中元素的数量