[LeetCode] 211. Add and Search Word - Data structure design

Problem

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 a-z.

Solution

class WordDictionary {
    public class TrieNode {
        public TrieNode[] children = new TrieNode[26];
        public boolean isWord = false;
    }
    
    TrieNode root = new TrieNode();
    
    public void addWord(String word) {
        TrieNode node = root;
        for (char ch: word.toCharArray()) {
            if (node.children[ch-'a'] == null) {
                node.children[ch-'a'] = new TrieNode();
            }
            node = node.children[ch-'a'];
        }
        node.isWord = true;
    }

    public boolean search(String word) {
        return helper(word, 0, root);
    }
    
    private boolean helper(String word, int start, TrieNode node) {
        if (start == word.length()) return node.isWord;
        char ch = word.charAt(start);
        if (ch == '.') {
            for (int i = 0; i < 26; i++) {
                if (node.children[i] != null && helper(word, start+1, node.children[i])) {
                    return true;
                }
            }
        } else {
            return node.children[ch-'a'] != null && helper(word, start+1, node.children[ch-'a']);
        }
        return false;
    }
}