数据结构与算法-双端队列

Gitee上开源的数据结构与算法代码库：数据结构与算法Gitee代码库

1. 概述

双端队列、队列、栈对比

	定义	特点
队列	一端删除（头）另一端添加（尾）	First In First Out
栈	一端删除和添加（顶）	Last In First Out
双端队列	两端都可以删除、添加
优先级队列		优先级高者先出队
延时队列		根据延时时间确定优先级
并发非阻塞队列	队列空或满时不阻塞
并发阻塞队列	队列空时删除阻塞、队列满时添加阻塞

注1：

• Java 中 LinkedList 即为典型双端队列实现，不过它同时实现了 Queue 接口，也提供了栈的 push pop 等方法

注2：

• 不同语言，操作双端队列的方法命名有所不同，参见下表

  操作	Java	JavaScript	C++	leetCode 641
  尾部插入	offerLast	push	push_back	insertLast
  头部插入	offerFirst	unshift	push_front	insertFront
  尾部移除	pollLast	pop	pop_back	deleteLast
  头部移除	pollFirst	shift	pop_front	deleteFront
  尾部获取	peekLast	at(-1)	back	getRear
  头部获取	peekFirst	at(0)	front	getFront

• 吐槽一下 leetCode 命名比较 low

• 常见的单词还有 enqueue 入队、dequeue 出队

2. 代码实现

a. 接口定义

/**
 * 双端队列接口
 * @param  队列中元素类型
 */
public interface Deque<E> {

    boolean offerFirst(E e);

    boolean offerLast(E e);

    E pollFirst();

    E pollLast();

    E peekFirst();

    E peekLast();

    boolean isEmpty();

    boolean isFull();
}

b. 基于双向环形链表实现

public class LinkedListDeque<E> implements Deque<E>, Iterable<E> {

    int capacity;
    int size;
    Node<E> sentinel = new Node<E>(null, null, null);

    static class Node<E>{
        Node<E> prev;
        E value;
        Node<E> next;

        public Node(Node<E> prev, E value, Node<E> next) {
            this.prev = prev;
            this.value = value;
            this.next = next;
        }
    }

    public LinkedListDeque(int capacity) {
        this.capacity = capacity;
        sentinel.next = sentinel;
        sentinel.prev = sentinel;
    }

    @Override
    public boolean offerFirst(E e) {
        if (isFull()){
            return false;
        }

        Node<E> prev = sentinel;
        Node<E> next = sentinel.next;
        Node<E> added = new Node<>(prev, e, next);
        prev.next = added;
        next.prev = added;
        size++;

        return true;
    }

    @Override
    public boolean offerLast(E e) {
        if (isFull()){
            return false;
        }

        Node<E> prev = sentinel.prev;
        Node<E> next = sentinel;
        Node<E> added = new Node<>(prev, e, next);
        prev.next = added;
        next.prev = added;
        size++;

        return true;
    }

    @Override
    public E pollFirst() {
        if (isEmpty()){
            return null;
        }

        Node<E> a = sentinel;
        Node<E> removed = sentinel.next;
        Node<E> b = removed.next;
        a.next = b;
        b.prev = a;
        size--;

        return removed.value;
    }

    @Override
    public E pollLast() {
        if (isEmpty()){
            return null;
        }

        Node<E> b = sentinel;
        Node<E> removed = sentinel.prev;
        Node<E> a = removed.prev;
        a.next = b;
        b.prev = a;
        size--;

        return removed.value;
    }

    @Override
    public E peekFirst() {
        if (isEmpty()){
            return null;
        }
        return sentinel.next.value;
    }

    @Override
    public E peekLast() {
        if (isEmpty()){
            return null;
        }
        return sentinel.prev.value;
    }

    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    @Override
    public boolean isFull() {
        return size == capacity;
    }

    @Override
    public Iterator<E> iterator() {
        return new Iterator<E>() {
            Node<E> p = sentinel.next;
            @Override
            public boolean hasNext() {
                return p != sentinel;
            }

            @Override
            public E next() {
                E value = p.value;
                p = p.next;
                return value;
            }
        };
    }   
}

c. 基于双向环形链表实现

/**
 * 基于循环数组实现 1
 * 特点：
 *  - tail 停下来的位置不存储，会浪费一个位置
 */
public class ArrayDeque<E> implements Deque<E>, Iterable<E> {

    /*
        h - head
        t - tail

        offerLast()  先添加到tail的位置，tail++
        offerFirst() head--, 添加到head的位置

        pollFirst() 先获取要移除的值，head++
        pollLast()  先tail-1，再获取值

        head == tail  空
        head ~ tail == 数组长度-1  满
     */

    E[] array;
    int head;
    int tail;

    @SuppressWarnings("all")
    public ArrayDeque(int capacity) {
        array = (E[]) new Object[capacity + 1];
    }

    /**
     * 指针 + 1 越界换算
     * @param i
     * @param length
     * @return
     */
    static int inc(int i, int length){
        if (i + 1 >= length){
            return 0;
        }
        return i + 1;
    }

    /**
     * 指针 - 1 越界换算
     * @param i
     * @param length
     * @return
     */
    static int dec(int i, int length){
        if (i - 1 < 0){
            return length - 1;
        }
        return i - 1;
    }

    @Override
    public boolean offerFirst(E e) {
        if (isFull()){
            return false;
        }
        head = dec(this.head, array.length);
        array[head] = e;
        return true;
    }

    @Override
    public boolean offerLast(E e) {
        if (isFull()){
            return false;
        }
        array[tail] = e;
        tail = inc(tail, array.length);
        return true;
    }

    @Override
    public E pollFirst() {
        if (isEmpty()){
            return null;
        }
        E value = array[head];
        array[head] = null; // help GC
        head = inc(head, array.length);
        return value;
    }

    @Override
    public E pollLast() {
        if (isEmpty()){
            return null;
        }
        tail = dec(tail, array.length);
        E value = array[tail];
        array[tail] = null; // help GC
        return value;
    }

    @Override
    public E peekFirst() {
        if (isEmpty()){
            return null;
        }
        return array[head];
    }

    @Override
    public E peekLast() {
        if (isEmpty()){
            return null;
        }
        return array[dec(tail, array.length)];
    }

    @Override
    public boolean isEmpty() {
        return head == tail;
    }

    @Override
    public boolean isFull() {
        if (tail > head){
            return tail - head == array.length - 1;
        } else if (tail < head){
            return head - tail == 1;
        } else {
            return false;
        }
    }

    @Override
    public Iterator<E> iterator() {
        return new Iterator<E>() {
            int p = head;
            @Override
            public boolean hasNext() {
                return p != tail;
            }

            @Override
            public E next() {
                E e = array[p];
                p = inc(p, array.length);
                return e;
            }
        };
    }
}