def add_string_helper(self, node, string, index):
if string is None or index >= len(string):
return None # terminating string or root
if node is None:
node = TrieNode('*')
character = string[index]
children = node.get_children()
if character not in node.get_children():
#print("adding new node", character, "root node", node.get_char())
children[character] = TrieNode(character)
new_node = self.add_string_helper(children[character], string, index+1)
if new_node is not None:
node.add_children(character, new_node)
return node
