LeetCode21-合并两个有序链表

递归实现

public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
       if (list1 == null) {
           return list2;
       } else if (list2 == null) {
           return list1;
       }

       if (list1.val < list2.val) {
           ListNode res = mergeTwoLists(list1.next, list2);
           list1.next = res;
           return list1;
       } else {
           ListNode res = mergeTwoLists(list1, list2.next);
           list2.next = res;
           return list2;
       }

   }