Add and Search Word - Data structure design

Design a data structure that supports the following two operations:

void addWord(word)
bool search(word)

search(word) can search a literal word or a regular expression string containing only letters a-z or .. A . means it can represent any one letter.

For example:

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

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

public class WordDictionary {
 
    TrieNode root = new TrieNode(' ');

    // Adds a word into the data structure.
    public void addWord(String word) {
        TrieNode node = root;
        for (int i = 0; i < word.length(); i++) {
            int cid = word.charAt(i) - 'a';
            if (node.children[cid] == null) {
                node.children[cid] = new TrieNode(word.charAt(i));
            }
            node = node.children[cid];
        }
        node.isWord = true;
    }

    // Returns if the word is in the data structure. A word could
    // contain the dot character '.' to represent any one letter.
    public boolean search(String word) {
        TrieNode node = root;
        return searchHelper(word, root);
    }
 
    private boolean searchHelper(String word, TrieNode node) {
        if (node == null) {
            return false;
        }
        if (word.isEmpty()) {
            return node != null && node.isWord;
        }
        if (word.charAt(0) == '.') {
            for (char c = 'a'; c <= 'z'; c++) {
                if (searchHelper(word.substring(1), node.children[c - 'a'])) {
                    return true;
                }
            }
            return false;
        } else {
            char c = word.charAt(0);
            return searchHelper(word.substring(1), node.children[c - 'a']);
        }
    }
 
    public static class TrieNode {
        char c;
        boolean isWord;
        TrieNode children[];
     
        public TrieNode(char c) {
            this.c = c;
            this.isWord = false;
            this.children = new TrieNode[26];
        }
    }
}

// Your WordDictionary object will be instantiated and called as such:
// WordDictionary wordDictionary = new WordDictionary();
// wordDictionary.addWord("word");
// wordDictionary.search("pattern");

===============

public class WordDictionary {
   
    TrieNode root = new TrieNode(' ');

    // Adds a word into the data structure.
    public void addWord(String word) {
        TrieNode node = root;
        for (int i = 0; i < word.length(); i++) {
            int cid = word.charAt(i) - 'a';
            if (node.children[cid] == null) {
                node.children[cid] = new TrieNode(word.charAt(i));
            }
            node = node.children[cid];
        }
        node.isWord = true;
    }

    // Returns if the word is in the data structure. A word could
    // contain the dot character '.' to represent any one letter.
   
    private String word;
   
    public boolean search(String word) {
        this.word = word;
        TrieNode node = root;
        return searchHelper(0, root);
    }
   
    private boolean searchHelper(int cur, TrieNode node) {
        if (node == null) {
            return false;
        }
        if (cur == word.length()) {
            return node != null && node.isWord;
        }
        if (word.charAt(cur) == '.') {
            for (char c = 'a'; c <= 'z'; c++) {
                if (searchHelper(cur + 1, node.children[c - 'a'])) {
                    return true;
                }
            }
            return false;
        } else {
            char c = word.charAt(cur);
            return searchHelper(cur + 1, node.children[c - 'a']);
        }
    }
   
    public static class TrieNode {
        char c;
        boolean isWord;
        TrieNode children[];
       
        public TrieNode(char c) {
            this.c = c;
            this.isWord = false;
            this.children = new TrieNode[26];
        }
    }
}

// Your WordDictionary object will be instantiated and called as such:
// WordDictionary wordDictionary = new WordDictionary();
// wordDictionary.addWord("word");
// wordDictionary.search("pattern");