2、数组模拟队列

AcWing 829. 模拟队列

import java.util.*;

//ACWing
public class Main {
    public static void main(String[] args) {
        Main main = new Main();
        main.duilie();

    }

    int N = 100010;
    int head = -1, tail = -1;
    int[] nums = new int[N];

    void push(int n) {
        nums[++tail] = n;
        if (head == -1) head++;
    }

    int query() {
        return nums[head];
    }

    int pop() {
        return nums[head++];
    }

    String empty() {
        if(head<=tail){
            return "NO";
        }else{
            return "YES";
        }
    }

    void duilie() {
        Scanner scanner = new Scanner(System.in);
        int M = scanner.nextInt();
        for (int i = 0; i < M; i++) {
            String op = scanner.next();
            if (op.equals("push")) {
                int n=scanner.nextInt();
                push(n);
            }
            if (op.equals("pop")) {
                pop();
            }
            if (op.equals("empty")) {
                System.out.println(empty());
            }
            if (op.equals("query")) {
                System.out.println(query());
            }
        }
    }
}