Merge Two Sorted Lists (Java)

题目：
Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.Example:Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4题目为：将两个有序链表排序
思路：
        先设置一个带头结点的链表L3.头节点的val值随意。依次比较L1和l2当前结点val值的大小。若L1的val值较小，就将L1的值赋给L3。L1、L3指针向后移动一位。若L2较小，同理。直到L1或者L2某一个链表为空。若L1为空，就将L3下一节点指向L1。L2为空，同理。返回时，返回L3头节点之后的节点。

代码：/**
* 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 l3 = new ListNode(0);
ListNode l4=l3;
//ListNode pListNode = l1;
//ListNode qListNode = l2;
if(l1==null){
return l2;
}
if(l2==null){
return l1;
}
while(l1!=null&&l2!=null){
if(l1.val<=l2.val){
l3.next = l1;
l3 = l3.next;
l1 = l1.next;
}else {
l3.next = l2;
l3 = l3.next;
l2 = l2.next;
}
}
if(l1==null){
l3.next = l2;
}
if(l2==null){
l3.next = l1;
}

return l4.next;

}
}