[LeetCode] Word Search II

A simple combination of Implement Trie (Prefix Tree) and Word Search. If you've solved them, this problem will become easy :-)

The following code is based on DFS and should be self-explanatory enough. Well, just go ahead and read it. It is long but clear :-)

class TrieNode {
public:
    bool isKey;
    TrieNode* children[26];
    TrieNode() : isKey(false) {
        memset(children, NULL, sizeof(TrieNode*) * 26);
    }
};

class Trie {
public:
    Trie() {
        root = new TrieNode();
    }
    
    void add(string& word) {
        TrieNode* run = root;
        for (char c : word) {
            if (!(run -> children[c - 'a']))
                run -> children[c - 'a'] = new TrieNode();
            run = run -> children[c - 'a'];
        }
        run -> isKey = true;
    }
    
    bool search(string& word) {
        TrieNode* p = query(word);
        return p && p -> isKey;
    }
    
    bool startsWith(string& prefix) {
        return query(prefix) != NULL;
    }
private:
    TrieNode* root;
    TrieNode* query(string& s) {
        TrieNode* run = root;
        for (char c : s) {
            if (!(run -> children[c - 'a'])) return NULL;
            run = run -> children[c - 'a'];
        }
        return run;
    }
};

class Solution {
public:
    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
        Trie trie = Trie();
        for (string word : words) trie.add(word);
        int m = board.size(), n = board[0].size();
        for (int r = 0; r < m; r++)
            for (int c = 0; c < n; c++)
                find(trie, board, "", r, c);
        return vector<string> (st.begin(), st.end());
    }
private:
    unordered_set<string> st;
    void find(Trie trie, vector<vector<char>>& board, string word, int r, int c) {
        int m = board.size(), n = board[0].size();
        if (board[r][c] == '%') return;
        word += board[r][c];
        if (!trie.startsWith(word)) return;
        if (trie.search(word)) st.insert(word);
        board[r][c] = '%';
        if (r) find(trie, board, word, r - 1, c);
        if (r < m - 1) find(trie, board, word, r + 1, c);
        if (c) find(trie, board, word, r, c - 1);
        if (c < n - 1) find(trie, board, word, r, c + 1);
        board[r][c] = word.back();
    }
};