两个有序的链表的合并

public class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
  }

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
       if(l1 == null) return l2;
       if(l2 == null) return l1;if(l1.val < l2.val){
           l1.next = mergeTwoLists(l1.next,l2);
           return l1;
        }else{
           l2.next = mergeTwoLists(l1,l2.next);
           return l2;
        }
    }
}