Implement Trie (Prefix Tree)

A trie (pronounced as “try”) or prefix tree is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. There are various applications of this data structure, such as autocomplete and spellchecker.

Implement the Trie class:

Trie() Initializes the trie object.
void insert(String word) Inserts the string word into the trie.
boolean search(String word) Returns true if the string word is in the trie (i.e., was inserted before), and false otherwise.
boolean startsWith(String prefix) Returns true if there is a previously inserted string word that has the prefix prefix, and false otherwise.

Example 1:

Input
["Trie", "insert", "search", "search", "startsWith", "insert", "search"]
[[], ["apple"], ["apple"], ["app"], ["app"], ["app"], ["app"]]
Output
[null, null, true, false, true, null, true]

Explanation
Trie trie = new Trie();
trie.insert("apple");
trie.search("apple");   // return True
trie.search("app");     // return False
trie.startsWith("app"); // return True
trie.insert("app");
trie.search("app");     // return True

Solution

We use a defaultdict to initialize a TrieNode, which contains a defaultdict of children characters. When we insert into the trie, we insert characters until we hit the end of characters, and then set the terminator to True. Same with searching, we traverse the trie until we reach the end of the word and see if there is a match.

class TrieNode:
    def __init__(self):
        self.children = defaultdict(TrieNode)
        self.is_terminator = False

class Trie:
    def __init__(self):
        self.root = TrieNode()

    def insert(self, word: str) -> None:
        curr = self.root
        for c in word:
            curr = curr.children[c]

        curr.is_terminator = True

    def search(self, word: str) -> bool:
        curr = self.root
        for c in word:
            if c in curr.children:
                curr = curr.children[c]
            else:
                return False

        return curr.is_terminator

    def startsWith(self, prefix: str) -> bool:
        curr = self.root
        for c in prefix:
            if c in curr.children:
                curr = curr.children[c]
            else:
                return False

        return True

Table of Contents

Implement Trie (Prefix Tree)

Solution