# 208. Implement Trie (Prefix Tree)

Implement a trie with insert, search, and startsWith methods.

Example:

Trie trie = new Trie();

trie.insert("apple");
trie.search("apple");   // returns true
trie.search("app");     // returns false
trie.startsWith("app"); // returns true
trie.insert("app");   
trie.search("app");     // returns true

Note:

  • You may assume that all inputs are consist of lowercase letters a-z.
  • All inputs are guaranteed to be non-empty strings.

# Solution

Approach 1: a trie (prefix tree) has an empty root node. Each subsequent node contains only 1 character.

# Code (Python)

Approach 1:

class TrieNode:
    
    def __init__(self):
        self.is_word = False
        self.children = {} # can also use an array of 26 chars

class Trie:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self._root = TrieNode()

    def insert(self, word: 'str') -> 'None':
        """
        Inserts a word into the trie.
        """
        cur = self._root
        for char in word:
            if char not in cur.children:
                cur.children[char] = TrieNode()
            cur = cur.children[char]
        cur.is_word = True

    def search(self, word: 'str') -> 'bool':
        """
        Returns if the word is in the trie.
        """
        cur = self._root
        for char in word:
            if char not in cur.children:
                return False
            cur = cur.children[char]
        return cur.is_word

    def startsWith(self, prefix: 'str') -> 'bool':
        """
        Returns if there is any word in the trie that starts with the given prefix.
        """
        cur = self._root
        for char in prefix:
            if char not in cur.children:
                return False
            cur = cur.children[char]
        return True

# Code (C++)

Approach 1:

class TrieNode {
public:
    TrieNode *children[26];
    bool isEnd;
    TrieNode() {
        for (int i = 0; i < 26; ++i)
        {
            children[i] = NULL;
        }
        isEnd = false;
    }
};

class Trie {
private:
    TrieNode *root;
public:
    /** Initialize your data structure here. */
    Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        TrieNode *node = root;
        for (int i = 0; i < word.size(); ++i)
        {
            int letterIdx = word[i] - 'a';
            if (node->children[letterIdx] == NULL)
            {
                node->children[letterIdx] = new TrieNode();
            }
            node = node->children[letterIdx];
        }
        node->isEnd = true;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        TrieNode *node = root;
        for (int i = 0; i < word.size(); ++i)
        {
            int letterIdx = word[i] - 'a';
            if (node->children[letterIdx] == NULL)
            {
                return false;
            }
            node = node->children[letterIdx];
        }
        return node->isEnd;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        TrieNode *node = root;
        for (int i = 0; i < prefix.size(); ++i)
        {
            int letterIdx = prefix[i] - 'a';
            if (node->children[letterIdx] == NULL)
            {
                return false;
            }
            node = node->children[letterIdx];
        }
        return true;
    }
};

/**
 * Your Trie object will be instantiated and called as such:
 * Trie* obj = new Trie();
 * obj->insert(word);
 * bool param_2 = obj->search(word);
 * bool param_3 = obj->startsWith(prefix);
 */