合併兩有序鏈表

struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};
class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)
    {
        //處理都爲空或一個爲空情況
       if(pHead1==NULL&&pHead2==NULL)   
           return NULL;
       if(pHead1==NULL)
           return pHead2;
       if(pHead2==NULL)
            return pHead1;

        ListNode* Head1=pHead1;
        ListNode* Head2=pHead2;
        ListNode* pNewHead=NULL;
        ListNode* pNewTail=NULL;

        //初始化新頭新尾
        if(Head1->val < Head2->val)
        {
            pNewHead=Head1;
            Head1=Head1->next;
        }
        else
        {
            pNewHead=Head2;
            Head2=pHead2->next;
        }
        pNewTail=pNewHead;

        //連接
        while(Head1 && Head2)
        {
            if(Head1->val < Head2->val)
            {
                pNewTail->next=Head1;
                Head1=Head1->next;
            }
            else
            {
                pNewTail->next=Head2;
                Head2=Head2->next;
            }
            pNewTail=pNewTail->next;
        }

        //一個爲空出循環則連接另一個
        if(Head1==NULL)
        {
            pNewTail->next=Head2;
        }
        else
        {
            pNewTail->next=Head1;
        }
        return pNewHead;  //返回鏈表頭
    }
};