merge two sorted lists， 合并两个有序序列

 /**
  * Definition for singly-linked list.
  * public class ListNode {
  *     int val;
  *     ListNode next;
  *     ListNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
         ListNode dumy = new ListNode(0);
         ListNode head = dumy;
         while(l1 != null && l2 != null)
         {
             if(l1.val < l2.val)
             {
                 dumy.next = l1;
                 l1 = l1.next;
             }
             else
             {
                 dumy.next = l2;
                 l2 = l2.next;
             }
             dumy = dumy.next;
         }
         if(l1 == null)
         {
             dumy.next = l2;
         }
         if(l2 == null)
         {
             dumy.next = l1;
         }
         return head.next;
     }
 }