15合并两个排序的链表

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};*/
class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2){
        if((pHead1 == nullptr) && (pHead2 == nullptr)){
            return nullptr;
        }
        if(pHead1 == nullptr){
            return pHead2;
        }
        if(pHead2 == nullptr){
            return pHead1;
        }
        ListNode* dummyhead = new ListNode(-1);
        ListNode* head = dummyhead;
        while(pHead1 != nullptr && pHead2 != nullptr){
            if(pHead1 -> val < pHead2 -> val){
                head -> next = pHead1;
                pHead1 = pHead1 -> next;
            }
            else{
                head -> next = pHead2;
                pHead2 = pHead2 -> next;
            }
            head = head -> next;
        }
        if(pHead1 != nullptr){
            head -> next = pHead1;
        }
        if(pHead2 != nullptr){
            head -> next = pHead2;
        }
        return dummyhead -> next;
    }
};