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.

class TrieNode {
    public char val;
    public boolean isWord;
    public TrieNode[] children = new TrieNode[26];

    public TrieNode() {
    }

    TrieNode(char c) {
        TrieNode node = new TrieNode();
        node.val = c;
    }
}

public class Trie {
    private TrieNode root;

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

    public void insert(String word) {
        TrieNode ws = root;
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            if (ws.children[c - 'a'] == null)
                ws.children[c - 'a'] = new TrieNode(c);
            ws = ws.children[c - 'a'];
        }
        ws.isWord = true;
    }

    public boolean search(String word) {
        TrieNode ws = root;
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            if (ws.children[c - 'a'] == null)
                return false;
            ws = ws.children[c - 'a'];
        }
        return ws.isWord;
    }

    public boolean startsWith(String prefix) {
        TrieNode ws = root;
        for (int i = 0; i < prefix.length(); i++) {
            char c = prefix.charAt(i);
            if (ws.children[c - 'a'] == null)
                return false;
            ws = ws.children[c - 'a'];
        }
        return true;
    }
}