注意点：递归实现

public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}
public class Solution {
   public ListNode Merge(ListNode list1,ListNode list2) {
            if(list1 == null)
                return list2;
            if(list2 == null)
                return list1;
            
            ListNode head = null;
            
            if(list1.val > list2.val)
            {
                head = list2;
                head.next = Merge(list1,list2.next);
            }else
            {
                head = list1;
                head.next = Merge(list1.next,list2);
            }
            return head;
     }
}