[LeetCode] Problem 208 - Implement Trie (Prefix Tree)

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

Example

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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.

Code

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public class TrieNode {
    private boolean word;
    private TrieNode[] children = new TrieNode[26];
}

private TrieNode root;

public Trie() {
    this.root = new TrieNode();
}

public void insert(String word) {
    TrieNode node = root;

    for (int i = 0; i < word.length(); i++) {
        int idx = word.charAt(i) - 'a';

        if (node.children[idx] == null)
            node.children[idx] = new TrieNode();

        node = node.children[idx];
    }

    node.word = true;
}

public boolean search(String word) {
    TrieNode node = find(word);
    return node != null && node.word;
}

public boolean startsWith(String prefix) {
    TrieNode node = find(prefix);
    return node != null;
}

private TrieNode find(String prefix) {
    TrieNode node = root;

    for (int i = 0; i < prefix.length(); i++) {
        int idx = prefix.charAt(i) - 'a';

        if (node.children[idx] == null)
            return null;

        node = node.children[idx];
    }

    return node;
}