LeetCode第641题：设计循环双端队列

题目详述

设计实现双端队列。
你的实现需要支持以下操作：

• MyCircularDeque(k)：构造函数,双端队列的大小为k。
• insertFront()：将一个元素添加到双端队列头部。 如果操作成功返回 true。
• insertLast()：将一个元素添加到双端队列尾部。如果操作成功返回 true。
• deleteFront()：从双端队列头部删除一个元素。 如果操作成功返回 true。
• deleteLast()：从双端队列尾部删除一个元素。如果操作成功返回 true。
• getFront()：从双端队列头部获得一个元素。如果双端队列为空，返回 -1。
• getRear()：获得双端队列的最后一个元素。 如果双端队列为空，返回 -1。
• isEmpty()：检查双端队列是否为空。
• isFull()：检查双端队列是否满了。

解法一

数组实现。时间复杂度：O(1)，空间复杂度：O(1)

class MyCircularDeque {
    private int capacity;
    private int[] arr;
    private int front; //注意计算的下标是绝对值
    private int rear;  //注意计算的下标是绝对值
    public MyCircularDeque(int k) {
        capacity = k + 1;
        arr = new int[capacity];
        front = 0;
        rear = 0;
    }
    
    public boolean insertFront(int value) {
        if (isFull()) return false;
        front = (front - 1 + capacity) % capacity;
        arr[front] = value;
        return true;
    }
   
    public boolean insertLast(int value) {
        if (isFull()) return false;
        arr[rear] = value;
        rear = (rear + 1) % capacity;
        return true;
    }
    
    public boolean deleteFront() {
        if (isEmpty()) return false;
        front = (front + 1) % capacity;
        return true;
    }
    
    public boolean deleteLast() {
        if (isEmpty()) return false;
        rear = (rear - 1 + capacity) % capacity;
        return true;
    }
    
    public int getFront() {
        if (isEmpty()) return -1;
        return arr[front];
    }
    
    /** Get the last item from the deque. */
    public int getRear() {
        if (isEmpty()) return -1;
        return arr[(rear - 1 + capacity) % capacity];
    }
    
    public boolean isEmpty() {
        return front == rear;
    }
    
    public boolean isFull() {
        return (rear + 1) % capacity == front;
    }
}

解法二

链表实现。时间复杂度：O(1),空间复杂度：O(1)

class MyCircularDeque {
    class DNode {
        int val;
        DNode next;
        DNode prev;
    }
    private int size;
    private int capacity;
    private DNode head,tail;

    /** Initialize your data structure here. Set the size of the deque to be k. */
    public MyCircularDeque(int k) {
        this.size = 0;
        capacity = k;
        head = new DNode();
        tail = new DNode();
        head.next = tail;
        tail.prev = head;
    }
    
    /** Adds an item at the front of Deque. Return true if the operation is successful. */
    public boolean insertFront(int value) {
        if (isFull()) return false;
        DNode temp = new DNode();
        temp.val = value;
        temp.prev = head;
        temp.next = head.next;
        head.next.prev = temp;
        head.next = temp;
        size ++;
        return true; 
    }
    
    /** Adds an item at the rear of Deque. Return true if the operation is successful. */
    public boolean insertLast(int value) {
        if (isFull()) return false;
        DNode temp = new DNode();
        temp.val = value;
        temp.next = tail;
        temp.prev = tail.prev;
        tail.prev.next = temp;
        tail.prev = temp;
        size ++;
        return true;
    }
    
    /** Deletes an item from the front of Deque. Return true if the operation is successful. */
    public boolean deleteFront() {
        if (isEmpty()) return false;
        DNode temp = new DNode();
        temp = head.next;
        head.next = temp.next;
        temp.next.prev = head;
        size --;
        return true;
    }
    
    /** Deletes an item from the rear of Deque. Return true if the operation is successful. */
    public boolean deleteLast() {
        if (isEmpty()) return false;
        DNode temp = new DNode();
        temp = tail.prev;
        tail.prev = temp.prev;
        temp.prev.next = tail;
        size --;
        return true;
    }
    
    /** Get the front item from the deque. */
    public int getFront() {
        if (isEmpty()) return -1;
        return head.next.val;
    }
    
    /** Get the last item from the deque. */
    public int getRear() {
        if (isEmpty()) return -1;
        return tail.prev.val;
    }
    
    /** Checks whether the circular deque is empty or not. */
    public boolean isEmpty() {
        return size == 0;
    }
    
    /** Checks whether the circular deque is full or not. */
    public boolean isFull() {
        return this.size == capacity;
    }
}