合并排序链表

 ListNode* Merge(ListNode* head1, ListNode* head2)
    {
        if (head1 == NULL)
            return head2;
        else if (head2 == NULL)
            return head1;
         
        ListNode* pMergeHead = NULL;
        if(head1->val > head2->val)
        {
            pMergeHead = head2;
            pMergeHead->next = Merge(head1,head2->next);
        }
        else
        {
            pMergeHead = head1;
            pMergeHead->next = Merge(head1->next,head2);
        }
         
        return pMergeHead;
    }