public boolean exist(final char[][] board, final String word) {

        if (word == null) {
            return true;
        }

        if (board == null) {
            return false;
        }

        if (board.length * board[0].length < word.length()) {
            return false;
        }

        final char ch = word.charAt(0);
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                if (board[i][j] == ch) {
                    // recursive method
                    if (search(board, word.substring(1), i, j)) {
                        return true;
                    }
                }

            }
        }

        return false;

    }