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PHP - Insert a new node at the start of the Linked List



In this method, a new node is inserted at the beginning of the linked list. For example - if the given List is 10->20->30 and a new element 100 is added at the start, the Linked List becomes 100->10->20->30.

Inserting a new node at the beginning of the Linked List is very easy. First, a new node with given element is created. It is then added before the head of the given Linked List that makes the newly added node to new head of the Linked List by changing the head pointer to point to the new node.

Linked List - Add Node At Start

The function push_front is created for this purpose. It is a 4-step process.

public function push_front($newElement) {
  
  //1. allocate a new node
  $newNode = new Node();
  
  //2. assign data element
  $newNode->data = $newElement;
  
  //3. make next node of new node as head
  $newNode->next = $this->head; 
  
  //4. make new node as head
  $this->head = $newNode;   
}

The below is a complete program that uses above discussed concept to insert new node at the start of the linked list.

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

class LinkedList {
  public $head;

  public function __construct(){
    $this->head = null;
  }
  
  //Add element at the start of the list
  public function push_front($newElement) {
    $newNode = new Node();
    $newNode->data = $newElement;
    $newNode->next = $this->head; 
    $this->head = $newNode;   
  }

  //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 at the start of the list.
$MyList->push_front(10);
$MyList->push_front(20);
$MyList->push_front(30);
$MyList->PrintList();
?>

The above code will give the following output:

The list contains: 30 20 10 

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