LeetCode: Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first
two lists.

/**
* 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) {
if(l1 == NULL || l2 == NULL)
return l1 == NULL ? l2 : l1;
ListNode *head = l1->val < l2->val ? l1 : l2;
ListNode *p1 = l1, *p2 = l2;
ListNode *pre = NULL, *cur = NULL;
while(p1 != NULL && p2 != NULL)
{
if(p1->val < p2->val)
{
cur = p1;
p1 = p1->next;
}
else
{
cur = p2;
p2 = p2->next;
}
if(pre != NULL)
pre->next = cur;
pre = cur;

}
cur->next = p1 == NULL ? p2 : p1;
return head;
}
};

Round 2: 

class Solution {
public:
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
ListNode *iter, *head;
iter = new ListNode(0);
if(l1 == NULL && l2 == NULL)
return NULL;
else if(l1 == NULL || l2 == NULL)
return l1 == NULL ? l2 : l1;
else
head = l1->val < l2->val ? l1 : l2;
while(l1 != NULL && l2 != NULL)
{
iter->next = l1->val < l2->val ? l1 : l2;
if(iter->next == l1)
l1 = l1->next;
else
l2 = l2->next;
iter = iter->next;
}
iter->next = l1 == NULL ? l2: l1;
return head;

}
};

Round 3:

class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
if(l1 == NULL || l2 == NULL)
return l1 == NULL ? l2 : l1;
ListNode *newHead = NULL;
if(l1->val > l2->val)
{
newHead = l2;
l2 = l2->next;
}
else
{
newHead = l1;
l1 = l1->next;
}
ListNode *iter = newHead;
while(l1 != NULL & l2 != NULL)
{
if(l1->val > l2->val)
{
iter->next = l2;
l2 = l2->next;
}
else
{
iter->next = l1;
l1 = l1->next;
}
iter = iter->next;
}
if(l1 == NULL)
{
iter->next = l2;
}
else
{
iter->next = l1;

}
return newHead;
}
};