题解 | 【模板】队列操作

public class Program {
    public static void Main() {
        string line = System.Console.ReadLine();
        int q = int.Parse(line);
        System.Collections.Generic.Queue<long> queue = new
        System.Collections.Generic.Queue<long>();

        for (int i = 0; i < q; i++) {
            string[] num = System.Console.ReadLine().Split();
            int op = int.Parse(num[0]);

            if (op == 1) {
                long x = long.Parse(num[1]);
                queue.Enqueue(x);
            } else if (op == 2) {
                if (queue.Count == 0) {
                    System.Console.WriteLine("ERR_CANNOT_POP");
                } else {
                    queue.Dequeue();
                }
            } else if (op == 3) {
                if (queue.Count == 0) {
                    System.Console.WriteLine("ERR_CANNOT_QUERY");
                } else {
                    System.Console.WriteLine(queue.Peek());
                }
            } else if (op == 4) {
                System.Console.WriteLine(queue.Count);
            }
        }
    }
}