18. 合并两个有序链表

<?php
Class ListNode {
    public $val = 0;
    public $next;
    public function __construct($val) {
        $this->val = $val;
    }
}
class Solution {
    public function mergeTwoLists(ListNode $l1, ListNode $l2) {
        if (!$l1) return $l2;
        if (!$l2) return $l1;
        $dummyHead = new ListNode(0);
        $current = $dummyHead;
        while ($l1 || $l2) {
            if (!$l1) {
                $current->next = $l2;
                break;
            }
            if (!$l2) {
                $current->next = $l1;
                break;
            }
            if ($l1->val < $l2->val) {
                $current->next = $l1;
                $current = $current->next;
                $l1 = $l1->next;
            } else {
                $current->next = $l2;
                $current = $current->next;
                $l2 = $l2->next;
            }
        }
        return $dummyHead->next;
    }
}
$l1 = new ListNode(1);
$l1->next = new ListNode(2);
$l1->next->next = new ListNode(4);
$l2 = new ListNode(1);
$l2->next = new ListNode(3);
$l2->next->next = new ListNode(4);
$cls = new Solution();
$r = $cls->mergeTwoLists($l1, $l2);
print_r($r);