實戰PHP數據結構基礎之雙鏈表

時間 2019-11-17

標籤實戰 php 數據結構基礎鏈表欄目 PHP 简体版

原文原文鏈接

什麼是雙鏈表？

上一篇實戰PHP數據結構基礎之單鏈表說到git

單鏈表由一個一個的做爲節點的對象構成的，每個節點都有指向下一個節點的指針，最後一個節點的指針域指向空。每一個節點能夠存儲任何數據類型。github

而雙鏈表每一個節點有兩個指針域，分別指向前驅和後繼節點。單鏈表是單向的，而雙鏈表是雙向的。web

常見操做

對雙鏈表咱們常見的操做有以下：算法

insert
insertBefore
insertAfter
insertAtFirst
insertAtLast
deleteFirst
deleteLast
delete
reverse
getNthNode
...

PHP語言實現

首先咱們根據定義實現一個雙鏈表的ListNode類。數據結構

class ListNode {
    public $data = null;
    public $next = null;
    public $prev = null;

    public function __construct(string $data) {
        $this->data = $data;
    }
}
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再來看鏈表類，首先須要3個私有屬性，分別是頭節點、尾巴節點和長度。數據結構和算法

class DoubleLinkedList {
    private $head = null;
    private $last = null;
    private $length = 0;
}
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接下來仍是老規矩，直接看如何實現第一個即經常使用的插入，這是是一個平均時間複雜度爲O(n)的操做。post

/** * 插入一個節點 * @param string|null $data * @return bool * complexity O(n) */
public function insert(string $data = null) {
    $newNode = new ListNode($data);
    if ($this->head) {
        $currentNode = $this->head;
        while ($currentNode) {
            if ($currentNode->next === null) {
                $currentNode->next = $newNode;
                $newNode->prev = $currentNode;
                $this->last = $newNode;
                $this->length++;
                return true;
            }

            $currentNode = $currentNode->next;
        }
    } else {
        $this->head = &$newNode;
        $this->last = $newNode;
        $this->length++;
        return true;
    }

}
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再來看如何刪除節點。優化

/** * 刪除一個節點 * @param string $data * @return bool|ListNode * complexity O(n) */
public function delete(string $query = null) {
    if ($this->head) {
        $currentNode = $this->head;
        $prevNode = null;
    
        while ($currentNode) {
            if ($currentNode->data === $query) {
                if ($nextNode = $currentNode->next) {
                    if ($prevNode) {
                        $prevNode->next = $nextNode;
                        $nextNode->prev = $prevNode;
                    } else {
                        $this->head = $nextNode;
                        $nextNode->prev = null;
                    }
    
                    unset($currentNode);
                } else {
                    if ($prevNode) {
                        $this->last = $prevNode;
                        $prevNode->next = null;
                        unset($currentNode);
                    } else {
                        $this->head = null;
                        $this->last = null;
                    }
                }
    
                $this->length--;
                return true;
            }
    
            $prevNode = $currentNode;
            $currentNode = $currentNode->next;
        }
    }

    return false;
}
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反轉雙鏈表也不是很複雜。this

public function reverse() {
    if ($this->head !== null) {

        if ($this->head->next !== null) {
            $reversedList = null;
            $currentNode = $this->head;

            while ($currentNode !== null) {
                $next = $currentNode->next;
                $currentNode->next = $reversedList;
                $currentNode->prev = $next;

                $reversedList = $currentNode;
                $currentNode = $next;

            }

            $this->last = $this->head;
            $this->head = $reversedList;
        }
    }
}
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