两个有序（升序）的单向链表合并成一个，并确保合并后的链表也是升序的

单向升序链表定义：

public class ListNode {

int val;

ListNode next;

ListNode(int x) { val = x; }

}

合并代码：
public class Solution {

public ListNode mergeTwoLists(ListNode l1, ListNodel2) {

if(l1 == null){

System.out.println("l1 == null");

return l2;

}

if(l2 == null){

System.out.println("l2 == null");

return l1;

}

ListNode node = l1;

ListNode node1 = l2;

ListNode mergeHead = null;

if(node.val < node1.val){

mergeHead = node;

mergeHead.next = mergeTwoLists(node.next,node1);

}else{

mergeHead = node1;

mergeHead.next = mergeTwoLists(node,node1.next);

}

return mergeHead;

}

}