PHP 合并2个链表

<?php
class ListNode{
    var $val;
    var $next = NULL;
    function __construct($x){
        $this->val = $x;
    }
}
function Merge($pHead1, $pHead2) {
    if ($pHead1===null) {
        return $pHead2;
    }
    if ($pHead2===null) {
        return $pHead1;
    }
    if ($pHead1->val < $pHead2->val) {
        $pHead1->next = Merge($pHead1->next, $pHead2);
        return $pHead1;
    }
    $pHead2->next = Merge($pHead1, $pHead2->next);
    return $pHead2;
}