[LC] 211. 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.

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);
  */