public void solve_bad(char[][] board) {

    // this solution got Stack Overflow Error
    // there could be some technique to optimize
    // when a position is set to false,
    // all the 'O' around it can also be set to false
    for (int i = 0 ; i < board.length ; i++) {
      for (int j = 0 ; j < board[0].length ; j++) {
        helper_BAD(board, i, j);
      }
    }
    // put back 'F' to 'O'
    for (int i = 0 ; i < board.length ; i++) {
      for (int j = 0 ; j < board[0].length ; j++) {
        if (board[i][j] == 'F') {
          board[i][j] = 'O';
        }
      }
    }
  }