21. 合并两个有序链表

class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if(list1 == null){
            return list2;
        }
        if(list2 == null){
            return list1;
        }
        ListNode res = list1.val < list2.val ? list1 : list2;
        res.next = mergeTwoLists(res.next, list1.val >= list2.val ? list1 : list2);
        return res;
    }
}