[LC] 211. Add and Search Word

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.

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

class WordDictionary {
    private TrieNode root;

    /** Initialize your data structure here. */
    public WordDictionary() {
        root = new TrieNode(); 
    }
    
    /** Adds a word into the data structure. */
    public void addWord(String word) {
        TrieNode cur = root;
        for (char c: word.toCharArray()) {
            if (cur.children[c - 'a'] == null) {
                cur.children[c - 'a'] = new TrieNode();
            }
            cur = cur.children[c - 'a'];
        }
        cur.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) {
        return helper(word, root, 0);
    }
    
    private boolean helper(String word, TrieNode root, int level) {
        if (level == word.length()) {
            return root.isWord;
        }
        char cur = word.charAt(level);
        if (cur == '.') {
            for (TrieNode child : root.children) {
                if (child != null && helper(word, child, level + 1)) {
                    return true;
                }
            }
            return false;
        } else {
            return root.children[cur - 'a'] != null && helper(word, root.children[cur - 'a'], level + 1);
        }
    }
}

class TrieNode {
    TrieNode[] children;
    boolean isWord;
    
    public TrieNode() {   
        children = new TrieNode[26];
        isWord = false;
    }
}

/**
 * Your WordDictionary object will be instantiated and called as such:
 * WordDictionary obj = new WordDictionary();
 * obj.addWord(word);
 * boolean param_2 = obj.search(word);
 */