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) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
if(l1 == NULL)
return l2;
if(l2 == NULL)
return l1;
ListNode *head = NULL;
if(l1->val <= l2->val){
head = l1;
head->next = mergeTwoLists(l1->next, l2);
}
else {
head = l2;
head->next =  mergeTwoLists(l1, l2->next);
}
return head;
}
};