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;

    }

};