合并两个有序链表（注意空指针异常）

将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

要注意判断两个结点是否为空结点，不然会出现空指针异常

/**
 * Definition for singly-linked list.
 * 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;
        ListNode l3=null;
        ListNode head=null;
        if(l1.val<=l2.val)
        {
            l3=l1;
            head=l3;
            l1=l1.next;
        }             
        else
        {
            l3=l2;
            head=l3;
            l2=l2.next;
        }
        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=l1;
        }
        else
        {
            l3.next=l2;
        }
        return head;
    }
}