Detailed explanation of the basics of PHP doubly linked list

<?php
/**
 * **双向链表
 * @author zhiyuan12@
 */
/**
 * 链表元素结点类
 */
class Node_Element {
  public $pre = NULL; // 前驱
  public $next = NULL; // 后继
  public $key = NULL; // 元素键值
  public $data = NULL; // 结点值
  function __Construct($key, $data) {
    $this->key = $key;
    $this->data = $data;
  }
}
/**
 * 双向链表类
 */
class DoubleLinkedList {
  private $head; // 头指针
  private $tail; // 尾指针
  private $current; // 当前指针
  private $len; // 链表长度
  function __Construct() {
    $this->head = self::_getNode ( null, null );
    $this->curelement = $this->head;
    $this->tail = $this->head;
    $len = 0;
  }
  /**
   * @ desc: 读取链表全部结点
   */
  public function readAll() {
    $tmp = $this->head;
    while ( $tmp->next !== null ) {
      $tmp = $tmp->next;
      var_dump ( $tmp->key, $tmp->data );
    }
  }
  public function move($pos1, $pos2) {
    $pos1Node = $this->findPosition ( $pos1 );
    $pos2Node = $this->findPosition ( $pos2 );
    if ($pos1Node !== null && $pos2Node !== null) {
      $tmpKey = $pos1Node->key;
      $tmpData = $pos1Node->data;
      $pos1Node->key = $pos2Node->key;
      $pos1Node->data = $pos2Node->data;
      $pos2Node->key = $tmpKey;
      $pos2Node->data = $tmpData;
      return true;
    }
    return false;
  }
  /**
   * @ desc: 在指定关键词删除结点
   *
   * @param : $key
   *     指定位置的链表元素key
   */
  public function delete($key) {
    $pos = $this->find ( $key );
    if ($pos !== null) {
      $tmp = $pos;
      $last = null;
      $first = true;
      while ( $tmp->next !== null && $tmp->next->key === $key ) {
        $tmp = $tmp->next;
        if (! $first) {
          $this->delNode ( $last );
        } else {
          $first = false;
        }
        $last = $tmp;
      }
      if ($tmp->next !== null) {
        $pos->pre->next = $tmp->next;
        $tmp->next->pre = $pos->pre;
      } else {
        $pos->pre->next = null;
      }
      $this->delNode ( $pos );
      $this->delNode ( $tmp );
    }
  }
  /**
   * @ desc: 在指定位置删除结点
   *
   * @param : $key
   *     指定位置的链表元素key
   */
  public function deletePosition($pos) {
    $tmp = $this->findPosition ( $pos );
    if ($tmp === null) {
      return true;
    }
    if ($tmp === $this->getTail ()) {
      $tmp->pre->next = null;
      $this->delNode ( $tmp );
      return true;
    }
    $tmp->pre->next = $tmp->next;
    $tmp->next->pre = $tmp->pre;
    $this->delNode ( $tmp );
  }
  /**
   * @ desc: 在指定键值之前插入结点
   *
   * @param : $key
   *     //指定位置的链表元素key
   * @param : $data
   *     //要插入的链表元素数据
   * @param : $flag
   *     //是否顺序查找位置进行插入
   */
  public function insert($key, $data, $flag = true) {
    $newNode = self::_getNode ( $key, $data );
    $tmp = $this->find ( $key, $flag );
    if ($tmp !== null) {
      $newNode->pre = $tmp->pre;
      $newNode->next = $tmp;
      $tmp->pre = $newNode;
      $newNode->pre->next = $newNode;
    } else {
      $newNode->pre = $this->tail;
      $this->tail->next = $newNode;
      $this->tail = $newNode;
    }
    $this->len ++;
  }
  /**
   * @ desc: 在指定位置之前插入结点
   *
   * @param : $pos
   *     指定插入链表的位置
   * @param : $key
   *     指定位置的链表元素key
   * @param : $data
   *     要插入的链表元素数据
   */
  public function insertPosition($pos, $key, $data) {
    $newNode = self::_getNode ( $key, $data );
    $tmp = $this->findPosition ( $pos );
    if ($tmp !== null) {
      $newNode->pre = $tmp->pre;
      $newNode->next = $tmp;
      $tmp->pre = $newNode;
      $newNode->pre->next = $newNode;
    } else {
      $newNode->pre = $this->tail;
      $this->tail->next = $newNode;
      $this->tail = $newNode;
    }
    $this->len ++;
    return true;
  }
  /**
   * @ desc: 根据key值查询指定位置数据
   *
   * @param : $key
   *     //指定位置的链表元素key
   * @param : $flag
   *     //是否顺序查找
   */
  public function find($key, $flag = true) {
    if ($flag) {
      $tmp = $this->head;
      while ( $tmp->next !== null ) {
        $tmp = $tmp->next;
        if ($tmp->key === $key) {
          return $tmp;
        }
      }
    } else {
      $tmp = $this->getTail ();
      while ( $tmp->pre !== null ) {
        if ($tmp->key === $key) {
          return $tmp;
        }
        $tmp = $tmp->pre;
      }
    }
    return null;
  }
  /**
   * @ desc: 根据位置查询指定位置数据
   *
   * @param : $pos
   *     //指定位置的链表元素key
   */
  public function findPosition($pos) {
    if ($pos <= 0 || $pos > $this->len)
      return null;
    if ($pos < ($this->len / 2 + 1)) {
      $tmp = $this->head;
      $count = 0;
      while ( $tmp->next !== null ) {
        $tmp = $tmp->next;
        $count ++;
        if ($count === $pos) {
          return $tmp;
        }
      }
    } else {
      $tmp = $this->tail;
      $pos = $this->len - $pos + 1;
      $count = 1;
      while ( $tmp->pre !== null ) {
        if ($count === $pos) {
          return $tmp;
        }
        $tmp = $tmp->pre;
        $count ++;
      }
    }
    return null;
  }
  /**
   * @ desc: 返回链表头节点
   */
  public function getHead() {
    return $this->head->next;
  }
  /**
   * @ desc: 返回链表尾节点
   */
  public function getTail() {
    return $this->tail;
  }
  /**
   * @ desc: 查询链表节点个数
   */
  public function getLength() {
    return $this->len;
  }
  private static function _getNode($key, $data) {
    $newNode = new Node_Element ( $key, $data );
    if ($newNode === null) {
      echo "new node fail!";
    }
    return $newNode;
  }
  private function delNode($node) {
    unset ( $node );
    $this->len --;
  }
}
$myList = new DoubleLinkedList ();
$myList->insert ( 1, "test1" );
$myList->insert ( 2, "test2" );
$myList->insert ( "2b", "test2-b" );
$myList->insert ( 2, "test2-c" );
$myList->insert ( 3, "test3" );
$myList->insertPosition ( 5, "t", "testt" );
$myList->readAll ();
echo "+++";
$myList->deletePosition(0);
$myList->readAll ();
echo "..." . $myList->getLength ();
var_dump ( $myList->findPosition ( 3 )->data );
?>

int(1)
string(5) "test1"
int(2)
string(7) "test2-c"
int(2)
string(5) "test2"
string(2) "2b"
string(7) "test2-b"
string(1) "t"
string(5) "testt"
int(3)
string(5) "test3"
+++int(1)
string(5) "test1"
int(2)
string(7) "test2-c"
int(2)
string(5) "test2"
string(2) "2b"
string(7) "test2-b"
string(1) "t"
string(5) "testt"
int(3)
string(5) "test3"
...6string(5) "test2"