17.合并两个排序的链表

当代码试图访问空指针指向的内存时程序就会崩溃，从而导致鲁棒性问题。所以要对空链表单独处理。

ListNode* Merge(ListNode* pHead1, ListNode* pHead2)

{

if (pHead1 == NULL)

return pHead2;

else if (pHead2 == NULL)

return pHead1;

ListNode* pMergedHead = NULL;

if (pHead1->m_nValue < pHead2->m_nValue)

{

pMergedHead = pHead1;

pMergedHead->m_pNext = Merge(pHead1->m_pNext,pHead2);

}

else

{

pMergedHead = pHead2;

pMergedHead->m_pNext = Merge(pHead1, pHead2->m_pNext);

}

return pMergedHead;

}