Trie树

class Trie {

    static class Node {
        boolean isWord;
        Node[] children;

        public Node() {
            isWord = false;
            children = new Node[26];
        }

        public Node(boolean isWord) {
            this.isWord = isWord;
            children = new Node[26];
        }
    }

    private Node root;

    /** Initialize your data structure here. */
    public Trie() {
        root = new Node();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        Node cur = root;
        for (int i = 0; i < word.length(); i++) {
            char ch = word.charAt(i);
            int idx = ch - 'a';
            if (cur.children[idx] == null) {
                cur.children[idx] = new Node();
            }
            cur = cur.children[idx];
            if (i == word.length() - 1) {
                cur.isWord = true;
            }
        }
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        Node cur = root;
        for (int i = 0; i < word.length(); i++) {
            char ch = word.charAt(i);
            int idx = ch - 'a';
            cur = cur.children[idx];
            if (cur == null) {
                return false;
            }
        }
        return cur.isWord;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String word) {
        Node cur = root;
        for (int i = 0; i < word.length(); i++) {
            char ch = word.charAt(i);
            int idx = ch - 'a';
            cur = cur.children[idx];
            if (cur == null) {
                return false;
            }
        }
        return true;
    }
}