/**
* 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 $list1
* @param ListNode $list2
* @return ListNode
*/
function mergeTwoLists($list1, $list2) {
}
}