22. k个一组翻转链表

<?php
Class ListNode {
    public $val = 0;
    public $next;
    public function __construct($val) {
        $this->val = $val;
    }
}
class Solution {
    public function reverseKGroup(ListNode $head, $k) {
        $node = $head;
        for($i = 0; $i < $k; $i++) {
            if (!$node) return $head;
            $node = $node->next;
        }
        $newHead = $this->reverse($head, $node);
        $head->next = $this->reverseKGroup($node, $k);
        return $newHead;
    }
    private function reverse(ListNode $start, ListNode $end) {
        $pre = null;
        while ($start !== $end) {
            $tmp = $start->next;
            $start->next = $pre;
            $pre = $start;
            $start = $tmp;
        }
        return $pre;
    }
}
$head = new ListNode(1);
$head->next = new ListNode(2);
$head->next->next = new ListNode(3);
$head->next->next->next = new ListNode(4);
$head->next->next->next->next = new ListNode(5);
$cls = new Solution();
$r = $cls->reverseKGroup($head, 3);
print_r($r);