What is the the difference between the following implementations of the length function?

	int length(List l)
	{
		int len = 0;
		
		while(l != NULL) {
			l = l->next;
			len++;
		}
		
		return len;
	}

	int length(List l)
	{
		if(l == NULL) return 0;
		return 1 + length(l->next);
	}

	int length(List l)
	{
		int length-helper(List l, int len)
		{
			if(l == NULL) return len;
			return length-helper(l->next, len+1);
		}
		
		return length-helper(l, 0);
	}