题解 | 【模板】队列操作

import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNextInt()) { // 注意 while 处理多个 case
            int n = in.nextInt();
            Queue<Integer> q = new LinkedList<>();
            for(int i = 0; i < n; i++) {
                int cur = in.nextInt();
                switch (cur) {
                    case 1:
                        q.add(in.nextInt());
                        break;
                    case 2:
                        if (q.isEmpty()) System.out.println("ERR_CANNOT_POP");
                        else q.poll();
                        break;
                    case 3:
                        if (q.isEmpty()) System.out.println("ERR_CANNOT_QUERY");
                        else System.out.println(q.peek());
                        break;
                    case 4:
                        System.out.println(q.size());
                }
            }
        }
    }
}