Leetcode 211: Add and Search Word - Data structure design

void addWord(word)
bool search(word)

addWord("bad")
addWord("dad")
addWord("mad")
search("pad") -> false
search("bad") -> true
search(".ad") -> true
search("b..") -> true

class TrieNode {
    public Character letter = null;
    // I just like to indicate whether a node is end or not, being explicit and clear, also it's good to know
    // if a word is actually a word or a substring of a long word
    public boolean isEnd = false;
    // This can be optimized by a 256 char array.
    public Map<Character, TrieNode> children = new HashMap<>();
}

public class AddAndSearchWord {
    private TrieNode root;
    public AddAndSearchWord() {
        this.root = new TrieNode();
    }

    // Adds a word into the data structure.
    public void addWord(String word) {
        if (word == null || word.length() == 0) {
            return;
        }

        this.insertHelper(this.root, word, 0);
    }

    // this can be done iteratively too
    private void insertHelper(TrieNode node, String word, int index) {
        if (index == word.length()) {
            node.isEnd = true;
            return;
        }

        char c = word.charAt(index);
        if (node.children.containsKey(c)) {
            insertHelper(node.children.get(c), word, index + 1);
            return;
        }

        TrieNode t = new TrieNode();
        t.letter = c;
        node.children.put(c, t);
        insertHelper(t, word, index + 1);
    }

    // Returns if the word is in the trie. A word could
    // contain the dot character '.' to represent any one and only one letter.
    public boolean search(String word) {
        if (word == null) {
            return false;
        }
        return this.searchHelper(this.root, word, 0);
    }

    // DFS to search the word, worst case is to search the entire trie space if all dots
    private boolean searchHelper(TrieNode node, String word, int index) {
        if (node == null) {
            return false;
        }
        // This is strict word matching, not including the substring, e.g., add("abcde"), search("abc") false, seasrch("abcde") true
        if (index == word.length()) {
            return node.isEnd;
        }

        char c = word.charAt(index);
        if (c != '.') {
            if (!node.children.containsKey(c)) {
                return false;
            }

            return searchHelper(node.children.get(c), word, index + 1);
        } else {
            for (Map.Entry<Character, TrieNode> entry : node.children.entrySet()) {
                // Don't need to go through a to z, just go through neighbours is enough, DFS
                if (searchHelper(entry.getValue(), word, index + 1)) {
                    return true;
                }
            }
            return false;
        }
    }
}