之前忘记记录这题了，现在补上。
合并两个有序的list，要求是：
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.
不知道我如下做是不是符合题目要求呢。
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2)
{
if (l1 == NULL) return l2;
if (l2 == NULL) return l1;
ListNode *tmp = new ListNode();
ListNode *head = tmp;
while(l1 && l2)
{
if (l1 -> val <= l2 -> val)
{
tmp -> next = l1;
tmp = l1;
l1 = l1 -> next;
}
else
{
tmp -> next = l2;
tmp = l2;
l2 = l2 -> next;
}
}
if (l1)
{
tmp -> next = l1;
}
else
{
tmp -> next = l2;
}
return head -> next;
}
};