Add and Search Word - Data structure design - LeetCode

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.

思路：构建字典树。搜索时，若当前字符为'.'，则将字典中当前位置存在的字符都搜索一遍。

class DictNode
{
public:
    DictNode *dict[27];
    DictNode ()
    {
        for (int i = 0; i < 27; i++)
            dict[i] = NULL;
    }
};
class WordDictionary {
public:
    WordDictionary()
    {
        root = new DictNode();
    }
    // Adds a word into the data structure.
    void addWord(string word) {
        DictNode *cur = root;
        for (int i = 0, len = word.size(); i < len; i++)
        {
            int loc = (int)(word[i] - 'a');
            if (cur->dict[loc] == NULL)
                cur->dict[loc] = new DictNode();
            cur = cur->dict[loc];
        }
        if (cur->dict[26] == NULL)
            cur->dict[26] = new DictNode();
    }
    bool help(DictNode *cur, string word)
    {
        if (cur == NULL) return false;
        if (word.size() == 0)
        {
            if (cur->dict[26] == NULL) return false;
            return true;
        }
        if (word[0] != '.')
        {
            int loc = (int)(word[0] - 'a');
            return help(cur->dict[loc], (word.size() > 1 ? word.substr(1) : ""));
        }
        else
        {
            for (int i = 0; i < 26; i++) if (cur->dict[i] != NULL)
            {
                if (help(cur->dict[i], (word.size() > 1 ? word.substr(1) : "")))
                    return true;
            }
            return false;
        }
    }

    // Returns if the word is in the data structure. A word could
    // contain the dot character '.' to represent any one letter.
    bool search(string word) {
        return help(root, word);
    }
private:
    DictNode *root;
};

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