208. Implement Trie (Prefix Tree)

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

Note:
You may assume that all inputs are consist of lowercase letters a-z.

class TrieNode {
  public char val;
  public boolean isWord;
  public TrieNode[] children;
  public TrieNode(char c) {
    this.val = c;
    this.isWord = false;
    this.children = new TrieNode[26];
  }
}

public class Trie {
  private TrieNode root;
  public Trie() {
    root = new TrieNode(' ');
  }
  
  public void insert(String word) {
    TrieNode tn = root;
    for (int i = 0; i < word.length(); i++) {
      char c = word.charAt(i);
      if (tn.children[c - 'a'] == null)
        tn.children[c - 'a'] = new TrieNode(c);
      tn = tn.children[c - 'a'];
    }
    tn.isWord = true;
  }
  
  public boolean search(String word) {
    TrieNode tn = root;
    for (int i = 0; i < word.length(); i++) {
      char c = word.charAt(i);
      if (tn.children[c - 'a'] == null)
        return false;
      tn = tn.children[c - 'a'];
    }
    return tn.isWord;
  }
  
  public boolean startsWith(String prefix) {
    TrieNode tn = root;
    for(int i = 0; i < prefix.length(); i++) {
      char c = prefix.charAt(i);
      if (tn.children[c - 'a'] == null)
        return false;
      tn = tn.children[c - 'a'];
    }
    return true;
  }
}