bool dfs(vector<vector<char> > &board, int r, int c, string word) {

   int m = board.size();

   int n = board[].size();

   int dir[][] = {{-, }, {, }, {, -}, {, }};

   if (word.empty())

     return true;

   if (r >=  && r < m && c >=  && c < n && board[r][c] == word[]) {

     for (int i = ; i < ; i++) {

       char tmp = board[r][c];

       board[r][c] = ;

       if (dfs(board, r + dir[i][], c + dir[i][], word.substr()))

         return true;

       board[r][c] = tmp;

     }

   }

   return false;

 }

 bool exist(vector<vector<char> > &board, string word) {

   if (board.empty() || board[].empty()) return false;

   int m = board.size();

   int n = board[].size();

   for (int i = ; i < m; i++)

     for (int j = ; j < n; j++)

       if (dfs(board, i, j, word))

         return true;

   return false;

 }