A simple Trie implementation

Trie.py

class Trie:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.root_dict = {}

    def insert(self, word):
        """
        Inserts a word into the trie.
        :type word: str
        :rtype: void
        """
        root = self.root_dict 
        for s in word:
            if s not in root:
                root[s] = {}
            root = root[s]    
        root['$'] = True

    def _search_prefix_helper(self, word):
        """
        Given a word returns False if the word is not a prefix of current wordlist or 
        it returns the root dictionary for the last letter of word found in the trie or it returns None if the prefix
        """
        root = self.root_dict 
        for s in word:
            if s not in root:
                return None
            root = root[s]
        return root

    def search(self, word):
        """
        Returns if the word is in the trie.
        :type word: str
        :rtype: bool
        """
        root = self._search_prefix_helper(word)
        return root is not None and '$' in root

    def startsWith(self, prefix):
        """
        Returns if there is any word in the trie that starts with the given prefix.
        :type prefix: str
        :rtype: bool
        """
        root = self._search_prefix_helper(prefix)
        return root is not None