题解 | 【模板】队列操作

import java.util.Scanner;
import java.util.*;

// 参考：https://blog.nowcoder.net/n/b52e09efd4324499aceaf9084d2b15a0
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        Queue<Integer> q =  new LinkedList<>();

        for(int i = 0;i<n;i++){
            int op = in.nextInt();
            switch(op){
                case 1:
                    int x = in.nextInt();
                    q.add(x);
                    break;
                case 2:
                    if(q.isEmpty()){
                        System.out.println("ERR_CANNOT_POP");
                    }else{
                        // 若队列非空，则仅将队头元素出队，否则输出 `ERR_CANNOT_POP`；
                        q.poll();
                    }
                    break;
                case 3:
                        // 查询并输出队首元素，队列为空时输出 `ERR_CANNOT_QUERY`；
                    if(q.isEmpty()){
                        System.out.println("ERR_CANNOT_QUERY");
                    }else{
                        System.out.println(q.peek());
                    }
                    break;
                case 4:
                    System.out.println(q.size());
                    break;
            }
        }
    }
}