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 lettersa-zor.. 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

Solution: 遇到“.”时，从a到z找

    TrieNode root;
    public WordDictionary() {
        root = new TrieNode();
    }

    /** Adds a word into the data structure. */
    public void addWord(String word) {
        TrieNode cur = root;
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            if (!cur.hashmap.containsKey(c)) {
                cur.hashmap.put(c, new TrieNode());
            }
            cur = cur.hashmap.get(c);
        }
        cur.hasWord = 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 search_helper(word, root, 0);
    }
    private boolean search_helper(String word, TrieNode cur, int index) {
            if (index == word.length()) {
                return cur.hasWord;
            }

            char c = word.charAt(index);
            if (c == '.') {
                for (char a = 'a'; a <= 'z'; a++) {
                    //不能写成if (cur.hashmap.containsKey(a)) return search_helper(word, cur.hashmap.get(a), index + 1);
                    if (cur.hashmap.containsKey(a) && search_helper(word, cur.hashmap.get(a), index + 1)) {
                        return true;
                    }
                }
                return false;
            } else if (cur.hashmap.containsKey(c)) {
                return search_helper(word, cur.hashmap.get(c), index + 1);
            } else {
                return false;
            }            
    }
    class TrieNode {
        boolean hasWord;
        Map<Character, TrieNode> hashmap;
        public TrieNode() {
            hashmap = new HashMap<>();
            hasWord = false;
        }
    }

或者：

    class TrieNode{
        TrieNode[] children;
        boolean hasWord;
        TrieNode(){
            children = new TrieNode[26];
            hasWord = false;
        }
    }
    public void addWord(String word) {
        TrieNode now = root;
        for(int i = 0; i < word.length(); i++){
            char c = word.charAt(i);
            if(now.children[c - 'a'] == null){
                now.children[c - 'a'] = new TrieNode();
            }
            now = now.children[c - 'a'];
        }
        now.hasWord = true;
    }