Merge Two Sorted Lists

ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) 

{

    if (l1 == NULL) return l2;

    if (l2 == NULL) return l1;

    ListNode *ret = NULL;

    if (l1->val < l2->val)

    {

        ret = l1;

        ret->next = mergeTwoLists(l1->next, l2);

    }

    else

    {

        ret = l2;

        ret->next = mergeTwoLists(l1, l2->next);

    }

    return ret;

}