合并两个排序链表

 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class  {
   
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null)
            return l2;
        if(l2==null)
            return l1;
        
        ListNode ans;
        if(l1.val<l2.val)
        {
            ans=l1;
            ans.next=mergeTwoLists(l1.next,l2);            
        }
            else
        {
            ans=l2;
            ans.next=mergeTwoLists(l1,l2.next);            
        }
        return ans;
    }
}