Leetcode Merge Two Sorted 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) {
// Note: The Solution object is instantiated only once and is reused by each test case.
if(l1 == NULL)  return l2;
if(l2 == NULL)  return l1;
ListNode *head = NULL, *cur = new ListNode(1);
while(l1 != NULL || l2 != NULL){
if(l1 == NULL || (l1 != NULL && l2 != NULL && l2->val <= l1->val)){
cur->next = new ListNode(l2->val);
l2 = l2->next;
}
else if(l2 == NULL || (l1 != NULL && l2 != NULL && l2->val > l1->val)){
cur->next = new ListNode(l1->val);
l1 = l1->next;
}
if(head == NULL)    head = cur->next;
cur = cur->next;
}
return head;
}
};