[C++]Merge Two Sorted Lists 归并两个排序的链表

leetcode 原题链接：https://leetcode.com/problems/merge-two-sorted-lists/

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.

简要翻译：归并两个已排序的链表，并作为一个新链表返回。新链表应该是原链表拼接而成的。即要在原链表上进行拼接操作。

简要分析：此题不难，但是要考虑到链表的特性，即链表是否为空的特性，还需注意指针的操作。

实现代码：

class Solution
{
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2)
{
if (l1 == NULL)
return l2;
if (l2 == NULL)
return l1;

ListNode *pHead = NULL;
ListNode *p = NULL;
while (l1 != NULL && l2 != NULL)
{
ListNode *temp = NULL;
if (l1->val < l2->val)
{
temp = l1;
l1 = l1->next;
} else
{
temp = l2;
l2 = l2->next;
}
if (pHead == NULL)
{
pHead = temp;
pHead->next = NULL;
p = pHead;
} else
{
p->next = temp;
p = p->next;
p->next = NULL;
}
}
if (l1 != NULL)
p->next = l1;
if (l2 != NULL)
p->next = l2;
return pHead;
}
};