Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list’s nodes, only nodes itself may be changed.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

/**
 * Definition for a singly-linked list.
 * class ListNode {
 *     public $val = 0;
 *     public $next = null;
 *     function __construct($val) { $this->val = $val; }
 * }
 */
class Solution {

    /**
     * @param ListNode $head
     * @return ListNode
     */
    function swapPairs($head) {
        if($head === null || $head->next === null){
            return $head;
        }
        
        $r = $head->next;
        $temp = $r->next;
        $r->next = $head;
        $r->next ->next = $this->swapPairs($temp);
        return $r;
    }
}