PHP - Swap node values of a Circular Singly Linked List

There are many instances where it is required to swap value of two nodes while working with a list. This can be achieved by traversing to the interested nodes and swap their values if the nodes are valid. For example - if the given list is 10->20->30->40->50. After swapping values of first and fourth nodes, the list will become 40->20->30->10->50.

The function swapNodeValues is created for this purpose which is a 4-step process.

public function swapNodeValues($node1, $node2) {
  
  //1. count the number of nodes in the list
  $temp = new Node();
  $temp = $this->head;
  $N = 0;
  if ($temp != null) {
    $N++;
    $temp = $temp->next;
  }
  while($temp != $this->head) {
    $N++;
    $temp = $temp->next;
  } 

  //2. check if the swap positions are valid entries
  if($node1 < 1 || $node1 > $N || $node2 < 1 || $node2 > $N)
    return;

  //3. traverse to the nodes where values to be swapped
  $pos1 = $this->head;
  $pos2 = $this->head;
  for($i = 1; $i < $node1; $i++) {
    $pos1 = $pos1->next;
  }
  for($i = 1; $i < $node2; $i++) {
    $pos2 = $pos2->next;
  }

  //4. swap the values of two nodes
  $val = $pos1->data;
  $pos1->data = $pos2->data;
  $pos2->data = $val;
}

The below is a complete program that uses above discussed concept to swap values of two given nodes of a circular singly linked list.

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

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;
    $newNode->next = null; 
    if($this->head == null) {
      $this->head = $newNode;
      $newNode->next = $this->head;
    } else {
      $temp = new Node();
      $temp = $this->head;
      while($temp->next !== $this->head) {
        $temp = $temp->next;
      }
      $temp->next = $newNode;
      $newNode->next = $this->head;
    }    
  }

  //swap node values
  public function swapNodeValues($node1, $node2) {
    
    $temp = new Node();
    $temp = $this->head;
    $N = 0;
    if ($temp != null) {
      $N++;
      $temp = $temp->next;
    }
    while($temp != $this->head) {
      $N++;
      $temp = $temp->next;
    } 

    if($node1 < 1 || $node1 > $N || $node2 < 1 || $node2 > $N)
      return;

    $pos1 = $this->head;
    $pos2 = $this->head;
    for($i = 1; $i < $node1; $i++) {
      $pos1 = $pos1->next;
    }
    for($i = 1; $i < $node2; $i++) {
      $pos2 = $pos2->next;
    }

    $val = $pos1->data;
    $pos1->data = $pos2->data;
    $pos2->data = $val;
  }   

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

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

//Add five elements in the list.
$MyList->push_back(10);
$MyList->push_back(20);
$MyList->push_back(30);
$MyList->push_back(40);
$MyList->push_back(50);

//Display the content of the list.
$MyList->PrintList();

//swap values of node=1 and node=4
$MyList->swapNodeValues(1, 4);

//Display the content of the list.
$MyList->PrintList();
?>

The above code will give the following output:

The list contains: 10 20 30 40 50 
The list contains: 40 20 30 10 50