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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if (l1 == NULL || l2 == NULL)
{
return l1 != NULL ? l1 : l2;
}
ListNode *l3 = NULL;
if (l1->val < l2->val)
{
l3 = l1;
l1 = l1->next;
}
else
{
l3 = l2;
l2 = l2->next;
}
ListNode *last = l3;
while (l1 != NULL && l2 != NULL)
{
if (l1->val < l2->val)
{
last->next = l1;
l1 = l1->next;
}
else
{
last->next = l2;
l2 = l2->next;
}
last = last->next;
}
l1 = l1 != NULL ? l1 : l2;
while (l1 != NULL)
{
last->next = l1;
last = last->next;
l1 = l1->next;
}
return l3;
}
};