[LeetCode]Merge Two Sorted Lists

struct ListNode {
int val;
ListNode *next;
ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
//this is a very simple implementation
//one pointer and one pointer's pointer
public:
ListNode * mergeTwoList(ListNode *l1, ListNode *l2) {
ListNode *h = NULL;
ListNode **p = &h;
while(l1 && l2)
{
if(l1->val < l2->val)
{
*p = l1;
l1 = l1->next;
}
else
{
*p = l2;
l2 = l2->next;
}
p = &((*p)->next);
}
if(l1) *p = l1;
else *p = l2;
return h;
}
};

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
ListNode dummy(-1);
ListNode* prev = &dummy;
while(l1 != NULL && l2 != NULL)
{
ListNode* tmp;
if(l1->val < l2->val) tmp = l1, l1 = l1->next;
else tmp = l2, l2 = l2->next;
prev->next = tmp;
prev = prev->next;
}
if(l1 == NULL && l2 == NULL) prev->next = NULL;
if(l1 != NULL) prev->next = l1;
if(l2 != NULL) prev->next = l2;

return dummy.next;
}
};