PHP - Insert a new node at a given position in the Doubly Linked List

In this method, a new element is inserted at the specified position in the doubly linked list. For example - if the given list is 10->20->30 and a new element 100 is added at position 2, the list becomes 10->100->20->30.

First, a new node with given element is created. If the insert position is 1, then the new node is made to head. Otherwise, traverse to the node that is previous to the insert position and check if it is null or not. In case of null, the specified position does not exist. In other case, update the links. The below figure describes the process, if the insert node is other than the head node.

The function push_at is created for this purpose. It is a 6-step process.

public function push_at($newElement, $position) {     
  
  //1. allocate node to new element
  $newNode = new Node(); 
  $newNode->data = $newElement;
  $newNode->next = null;
  $newNode->prev = null;

  //2. check if the position is > 0
  if($position < 1) {
    echo "\nposition should be >= 1.";
  } else if ($position == 1) {
    
    //3. if the position is 1, make new node as head
    $newNode->next = $this->head;
    $this->head->prev = $newNode;
    $this->head = $newNode;
  } else {
    
    //4. Else, make a temp node and traverse to the 
    //   node previous to the position
    $temp = new Node();
    $temp = $this->head;
    for($i = 1; $i < $position-1; $i++) {
      if($temp != null) {
        $temp = $temp->next;
      }
    }
 
    //5. If the previous node is not null, adjust 
    //   the links
    if($temp != null) {
      $newNode->next = $temp->next;
      $newNode->prev = $temp;
      $temp->next = $newNode; 
      if($newNode->next != null)
        $newNode->next->prev = $newNode; 
    } else {
      
      //6. When the previous node is null
      echo "\nThe previous node is null.";
    }       
  }
}

The below is a complete program that uses above discussed concept to insert new node at a given position in the doubly linked list.

<?php
//node structure
class Node {
  public $data;
  public $next;
  public $prev;
}

class LinkedList {
  public $head;

  public function __construct(){
    $this->head = null;
  }
  
  //Add new element at the end of the list
  public function push_back($newElement) {
    $newNode = new Node();
    $newNode->data = $newElement;
    $newNode->next = null;
    $newNode->prev = null; 
    if($this->head == null) {
      $this->head = $newNode;
    } else {
      $temp = new Node();
      $temp = $this->head;
      while($temp->next != null) {
        $temp = $temp->next;
      }
      $temp->next = $newNode;
      $newNode->prev = $temp;
    }    
  }

  //Inserts a new element at the given position
  public function push_at($newElement, $position) {     
    $newNode = new Node(); 
    $newNode->data = $newElement;
    $newNode->next = null;
    $newNode->prev = null;
    if($position < 1) {
      echo "\nposition should be >= 1.";
    } else if ($position == 1) {
      $newNode->next = $this->head;
      $this->head->prev = $newNode;
      $this->head = $newNode;
    } else {
      $temp = new Node();
      $temp = $this->head;
      for($i = 1; $i < $position-1; $i++) {
        if($temp != null) {
          $temp = $temp->next;
        }
      }
      if($temp != null) {
        $newNode->next = $temp->next;
        $newNode->prev = $temp;
        $temp->next = $newNode; 
        if($newNode->next != null)
          $newNode->next->prev = $newNode; 
      } else {
        echo "\nThe previous node is null.";
      }       
    }
  }  

  //display the content of the list
  public function PrintList() {
    $temp = new Node();
    $temp = $this->head;
    if($temp != null) {
      echo "The list contains: ";
      while($temp != null) {
        echo $temp->data." ";
        $temp = $temp->next;
      }
      echo "\n";
    } else {
      echo "The list is empty.\n";
    }
  }   
};

// test the code  
$MyList = new LinkedList();

//Add three elements in the list.
$MyList->push_back(10);
$MyList->push_back(20);
$MyList->push_back(30);
$MyList->PrintList();

//Insert an element at position 2
$MyList->push_at(100, 2);
$MyList->PrintList();

//Insert an element at position 1
$MyList->push_at(200, 1);
$MyList->PrintList();  
?>

The above code will give the following output:

The list contains: 10 20 30
The list contains: 10 100 20 30
The list contains: 200 10 100 20 30