leetcode 21.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.

解决方案：276ms

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode newList = head;
        
        if(l1 == null || l2 == null){//两者有为空的情况
            if(l1==null)
                return l2;
            if(l2 == null)
                return l1;
        }
        
        while(l1!=null && l2!=null){//两者都不为空
            
            if(l1.val <= l2.val){
                newList.next = l1;
                l1 = l1.next;
            }else{// if(l1.val < l2.val)
                newList.next = l2;
                l2 = l2.next;
            }
            newList = newList.next;
        }
        while(l1!=null){
            newList.next = l1;
            l1 = l1.next;
            newList = newList.next;
        }
        while(l2!=null){
            newList.next = l2;
            l2 = l2.next;
            newList = newList.next;
        }
        
        return head.next;
    }
}

总结：

　　这道题目很简单，就是我们以前数据结构学习的时候的一道课本上的算法代码，其实思路就在那里，很容易就想到，但是我这里犯错了，我在第一个while中把nextList = nextList.next放到了while第一句里面，当然下面的代码也就是nextList = l1;这样了，但是这样是不对的，具体原因我也没弄清楚，希望有人帮我解决问题。多谢诸位。

贴错误代码：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode newList = head;
        
        if(l1 == null || l2 == null){//两者有为空的情况
            if(l1==null)
                return l2;
            if(l2 == null)
                return l1;
        }
        
        while(l1!=null && l2!=null){//两者都不为空
            newList = newList.next;
            if(l1.val <= l2.val){
                newList = l1;
                l1 = l1.next;
            }else{// if(l1.val < l2.val)
                newList = l2;
                l2 = l2.next;
            }
            
        }
        while(l1!=null){
            newList.next = l1;
            l1 = l1.next;
            newList = newList.next;
        }
        while(l2!=null){
            newList.next = l2;
            l2 = l2.next;
            newList = newList.next;
        }
        
        return head.next;
    }
}

代码提示错误：

Input:     {2}, {1}
Output:    {}
Expected:  {1,2}

纠结，这个问题都没搞懂。。。