QueueArrayList

import java.util.*;

/*
  1. 双链表，head 用来删除， tail用来添加，
  2. 同时 head tail用来表示 位置 

 * design a Queue with arrayList, 但是有限制条件, arraylist的长度最多为10, queue不限制长度
 * 简单点儿方法可以把每个array最后一个item存下一个array的指针，这样每个size为10的array可以存9个元素。在空间上没有很高效，但是实现起来容易一些。
 */
public class QueueArrayList {
	static final int SIZE = 5;
	int head = 0, tail = 0;
	int count =0;
	List<Object> headList = new ArrayList<>();
	List<Object> tailList =this.headList;
	public void enqueue(Integer num) {
		if(tail==SIZE-1) {
				List<Object> newList = new ArrayList<>();
				newList.add(num);
				tailList.add(newList);
				tailList = (List<Object>)tailList.get(tail);
			tail = 0;
		}else {
			tailList.add(num);
		}
		tail++;
		count++;
	}
	
	public Integer peek() {
		if(count ==0) return null;
		return (Integer) headList.get(head);
	}
	
	public int size() {
		return count;
	}
	
	public Integer pop() {
		if(count==0) return null;
		int num = (int) headList.get(head);
		head++;
		count--;
		if(head==SIZE-1) {
			List<Object> newList = (List<Object>) headList.get(head);
			headList.clear();
			headList = newList;
			head=0;
		}
		return num;
	}
	
	public static void main(String[] args) {
		QueueArrayList qal = new QueueArrayList();
		qal.enqueue(1);
		qal.enqueue(2);
		qal.enqueue(3);
		qal.enqueue(4);
		qal.enqueue(5);
		qal.enqueue(6);
		qal.enqueue(7);
		System.out.println(qal.peek());
		System.out.println(qal.pop());
		System.out.println(qal.peek());
		System.out.println(qal.pop());
		System.out.println(qal.pop());
		//System.out.println(qal.pop());
		qal.enqueue(8);
		qal.enqueue(9);
		qal.enqueue(10);
		System.out.println(qal.peek());
		System.out.println(qal.pop());
		System.out.println(qal.peek());
	}
}