数据结构模拟实现Queue队列（双链表模拟）

本文介绍: 如果不为空，就判断head.next为不为空，也就是判断队列是不是只有一个元素，如果只有一个元素，就取出头结点的val值，再把head和last置为空，usedSize–；如果有多个元素，就取出头结点的val值，把头结点的next置为空，头结点往后走一步，头结点的prev置空，usedSize–。此方法是入队列方法，首先要创建一个链表node，判断链表中为不为空，如果是空，就把head和last定义成node，usedSize++；出队列三次，把队列出完，如果里面没元素了，就会抛异常。

一、队列的概念

public class MyQueue implements IQueue{
    class ListNode {
        public int value;
        public ListNode prev;
        public ListNode next;
        public ListNode(int value) {
            this.value = value;
        }
    }
    ListNode head;
    ListNode last;
    int usedSize;
}

public interface IQueue {
    public boolean offer(int x);
    public int poll();
    public int peek();
    public int size();
    public boolean isEmpty();
}

    public boolean offer(int x) {
        ListNode node = new ListNode(x);
        if(head == null) {
            head = node;
            last = node;
        } else {
            last.next = node;
            node.prev = last;
            last = last.next;
        }
        usedSize++;
        return true;
    }

    public int poll() {
        if(head == null) {
            //队列为空，出不了队列，抛异常
            throw new EmptyException("队列为空");
        }
        if(head.next == null) {
            int retValue = head.value;
            last = null;
            head = null;
            usedSize--;
            return retValue;
        }
        int retValue = head.value;
        ListNode nodeNext = this.head.next;
        head.next = null;
        nodeNext.prev = null;
        head = nodeNext;
        usedSize--;
        return retValue;
    }

//自定义异常类
public class EmptyException extends RuntimeException{
    public EmptyException(String msg) {
        super(msg);
    }
}

    public int peek() {
        if(head == null) {
            //队列为空，出不了队列，抛异常
            throw new EmptyException("队列为空");
        } else {
            int retValue = head.value;
            return retValue;
        }
    }

//自定义异常类
public class EmptyException extends RuntimeException{
    public EmptyException(String msg) {
        super(msg);
    }
}

    public int size() {
        return usedSize;
    }

    public boolean isEmpty() {
        return this.head == null;
    }

//自定义接口
public interface IQueue {
    public boolean offer(int x);
    public int poll();
    public int peek();
    public int size();
    public boolean isEmpty();
}

//MyQueue类
public class MyQueue implements IQueue{
    class ListNode {
        public int value;
        public ListNode prev;
        public ListNode next;
        public ListNode(int value) {
            this.value = value;
        }
    }
    ListNode head;
    ListNode last;
    int usedSize;

    @Override
    public boolean offer(int x) {
        ListNode node = new ListNode(x);
        if(head == null) {
            head = node;
            last = node;
        } else {
            last.next = node;
            node.prev = last;
            last = last.next;
        }
        usedSize++;
        return true;
    }

    @Override
    public int poll() {
        if(head == null) {
            //队列为空，出不了队列，抛异常
            throw new EmptyException("队列为空");
        }
        if(head.next == null) {
            int retValue = head.value;
            last = null;
            head = null;
            usedSize--;
            return retValue;
        }
        int retValue = head.value;
        ListNode nodeNext = this.head.next;
        head.next = null;
        nodeNext.prev = null;
        head = nodeNext;
        usedSize--;
        return retValue;
    }

    @Override
    public int peek() {
        if(head == null) {
            //队列为空，出不了队列，抛异常
            throw new EmptyException("队列为空");
        } else {
            int retValue = head.value;
            return retValue;
        }
    }

    @Override
    public int size() {
        return usedSize;
    }

    @Override
    public boolean isEmpty() {
        return this.head == null;
    }
}

//自定义异常类
public class EmptyException extends RuntimeException{
    public EmptyException(String msg) {
        super(msg);
    }
}