题解 | #用递归函数和栈逆序一个栈#

import java.util.Deque;
import java.util.Scanner;
import java.util.LinkedList;

public class Main{
    public static void main(String[] args)
    {
        myQueue myque = new myQueue();
        Scanner  sc = new Scanner(System.in);
        int length = Integer.parseInt(sc.nextLine());
        for(int i=0; i<length; i++)
        {
            String[] str = sc.nextLine().split(" ");
            if(str[0].equals("add"))
            {
                myque.add(Integer.parseInt(str[1]));
            }
            
            if(str[0].equals("peek"))
            {
                int num = myque.peek();
                System.out.println(num);
            }
            if(str[0].equals("poll"))
            {
                myque.poll();
            }  
        }
    }     
}

class myQueue{
    Deque<Integer> deque = null;
    Deque<Integer> deque_bgk = null;
    int top = 0; //栈顶元素
    public myQueue(){
        deque = new LinkedList<>();
        deque_bgk = new LinkedList<>();
    }
    public void add(int num){
        if(deque.isEmpty())
        {
            top = num;
        }
        deque.push(num);
    }
    
    public int peek(){
        return top;
    }
    
    public void poll(){
        int size = deque.size();
        while(--size>0){
            deque_bgk.push(deque.poll());
        }
        deque.poll();
        if(!deque_bgk.isEmpty())
            top = deque_bgk.peek();
        else
            top = 0;
        while(!deque_bgk.isEmpty()){
            deque.push(deque_bgk.poll());
        } 
    }
}