Swap Nodes in Pairs
<p>Given a&nbsp;linked list, swap every two adjacent nodes and return its head. You must solve the problem without&nbsp;modifying the values in the list&#39;s nodes (i.e., only nodes themselves may be changed.)</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">head = [1,2,3,4]</span></p>

<p><strong>Output:</strong> <span class="example-io">[2,1,4,3]</span></p>

<p><strong>Explanation:</strong></p>

<p><img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0000-0099/0024.Swap%20Nodes%20in%20Pairs/images/swap_ex1.jpg" style="width: 422px; height: 222px;" /></p>
</div>

<p><strong class="example">Example 2:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">head = []</span></p>

<p><strong>Output:</strong> <span class="example-io">[]</span></p>
</div>

<p><strong class="example">Example 3:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">head = [1]</span></p>

<p><strong>Output:</strong> <span class="example-io">[1]</span></p>
</div>

<p><strong class="example">Example 4:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">head = [1,2,3]</span></p>

<p><strong>Output:</strong> <span class="example-io">[2,1,3]</span></p>
</div>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the&nbsp;list&nbsp;is in the range <code>[0, 100]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 100</code></li>
</ul>
Recursion; Linked List
# Definition for singly-linked list.
# class ListNode {
#    public $val;
#    public $next;
#    public function __construct($val = 0, $next = null)
#    {
#        $this->val = $val;
#        $this->next = $next;
#    }
# }

class Solution {
    /**
     * @param ListNode $head
     * @return ListNode
     */

    function swapPairs($head) {
        $dummy = new ListNode(0);
        $dummy->next = $head;
        $prev = $dummy;

        while ($head !== null && $head->next !== null) {
            $first = $head;
            $second = $head->next;

            $first->next = $second->next;
            $second->next = $first;
            $prev->next = $second;

            $prev = $first;
            $head = $first->next;
        }

        return $dummy->next;
    }
}
