Reverse Nodes in k-Group
<p>Given the <code>head</code> of a linked list, reverse the nodes of the list <code>k</code> at a time, and return <em>the modified list</em>.</p>

<p><code>k</code> is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of <code>k</code> then left-out nodes, in the end, should remain as it is.</p>

<p>You may not alter the values in the list&#39;s nodes, only nodes themselves may be changed.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0000-0099/0025.Reverse%20Nodes%20in%20k-Group/images/reverse_ex1.jpg" style="width: 542px; height: 222px;" />
<pre>
<strong>Input:</strong> head = [1,2,3,4,5], k = 2
<strong>Output:</strong> [2,1,4,3,5]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0000-0099/0025.Reverse%20Nodes%20in%20k-Group/images/reverse_ex2.jpg" style="width: 542px; height: 222px;" />
<pre>
<strong>Input:</strong> head = [1,2,3,4,5], k = 3
<strong>Output:</strong> [3,2,1,4,5]
</pre>

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

<ul>
	<li>The number of nodes in the list is <code>n</code>.</li>
	<li><code>1 &lt;= k &lt;= n &lt;= 5000</code></li>
	<li><code>0 &lt;= Node.val &lt;= 1000</code></li>
</ul>

<p>&nbsp;</p>
<p><strong>Follow-up:</strong> Can you solve the problem in <code>O(1)</code> extra memory space?</p>
Recursion; Linked List
/**
 * Definition for a singly-linked list.
 * class ListNode {
 *     public $val = 0;
 *     public $next = null;
 *     function __construct($val = 0, $next = null) {
 *         $this->val = $val;
 *         $this->next = $next;
 *     }
 * }
 */
class Solution {
    /**
     * @param ListNode $head
     * @param Integer $k
     * @return ListNode
     */
    function reverseKGroup($head, $k) {
        $dummy = new ListNode(0);
        $dummy->next = $head;
        $pre = $dummy;

        while ($pre !== null) {
            $cur = $pre;
            for ($i = 0; $i < $k; $i++) {
                if ($cur->next === null) {
                    return $dummy->next;
                }
                $cur = $cur->next;
            }

            $node = $pre->next;
            $nxt = $cur->next;
            $cur->next = null;
            $pre->next = $this->reverse($node);
            $node->next = $nxt;
            $pre = $node;
        }

        return $dummy->next;
    }

    /**
     * Helper function to reverse a linked list.
     * @param ListNode $head
     * @return ListNode
     */
    function reverse($head) {
        $prev = null;
        $cur = $head;
        while ($cur !== null) {
            $nxt = $cur->next;
            $cur->next = $prev;
            $prev = $cur;
            $cur = $nxt;
        }
        return $prev;
    }
}
