leetcode Merge Two Sorted Lists

class Solution
{
    public:
      ListNode* mergeTwoLists(ListNode* l1, ListNode* l2)
      {
          if (l1 == NULL)
          {
             return l2;
          }

          if (l2 == NULL)
          {
              return l1;
          }
          ListNode* help = new ListNode(0);
          ListNode* head = help;

          while (l1 && l2)
          {
              if (l1->val > l2->val)
              {
                  help->next = l2;
                  l2 = l2->next;

              }
              else
              {
                  help->next = l1;
                  l1 = l1->next;              
              }

              help = help->next;
          }
          if (l1) 
          {
              help->next = l1;
          }
          if (l2)
          {
              help->next = l2;
          }

          return head->next;

      }
};

网上的用递归的解法：

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;
}