剑指offer--23.合并两个排序的链表

class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)
    {
         if(pHead1 == NULL && pHead2 == NULL) return NULL;
         if(pHead1 == NULL && pHead2 != NULL) return pHead2;
         if(pHead1 != NULL && pHead2 == NULL) return pHead1;
        
        if(pHead1->val > pHead2->val) {
            pHead2->next = Merge(pHead1, pHead2->next);
        } else {
            pHead1->next = Merge(pHead2, pHead1->next);
        }
        return pHead1;
    }
};