211. Design Add and Search Words Data Structure

# Medium

Key idea: very similar to #208, the only difference is search(), must have a helper function to do backtracking, check one letter every time until done.

C++

class WordNode {
public:
    WordNode *child[26];
    bool isEndOfWord;
    
    WordNode(): isEndOfWord(false) {
        for(auto &a : child) {
            a = nullptr;
        }
    }
};

class WordDictionary {
public:
    /** Initialize your data structure here. */
    WordDictionary() {
        root = new WordNode();
    }
    
    /** Adds a word into the data structure. */
    void addWord(string word) {
        WordNode *p = root;
        for(auto &a : word) {
            int i = a - 'a';
            if(!p->child[i]) p->child[i] = new WordNode();
            p = p->child[i];
        }
        p->isEndOfWord = true;
    }
    
    /** 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 helper(word, 0, root);
    }
    
private:
    WordNode *root;
    bool helper(string &word, int x, WordNode *p) {
        // edge case
        if(x == word.size()) return p->isEndOfWord;
        
        // regular case
        // is '.', backtracking
        if(word[x] == '.') {
            for(auto &a : p->child) {
                // return helper(word, x+1, a);
                if(a && helper(word, x+1, a)) return true;
            }
            return false;
        }
        // is letter
        else {
            return p->child[word[x] - 'a'] && helper(word, x+1, p->child[word[x] - 'a']);
        }  
    }
};






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