Question

In this great book, we’re asked to design an algorithm to figure out if someone has won in a game of tic-tac-toe.
The following solution is given, after which the author said:

Note that the runtime could be reduced to O(N) with the addition of
row and column count arrays (and two sums for the diagonals)

I tried hard, but I couldn’t figure out the meaning of that remark. How are these arrays and sums added ? Thanks !

enum Piece { Empty, Red, Blue };
enum Check { Row, Column, Diagonal, ReverseDiagonal }

Piece getIthColor(Piece[][] board, int index, int var, Check check) {
  if (check == Check.Row) return board[index][var];
  else if (check == Check.Column) return board[var][index];
  else if (check == Check.Diagonal) return board[var][var];
  else if (check == Check.ReverseDiagonal)      
    return board[board.length - 1 - var][var];      

  return Piece.Empty;
}

Piece getWinner(Piece[][] board, int fixed_index, Check check) {    
  Piece color = getIthColor(board, fixed_index, 0, check);
  if (color == Piece.Empty) return Piece.Empty;
  for (int var = 1; var < board.length; var++) {
    if (color != getIthColor(board, fixed_index, var, check)) {
      return Piece.Empty;
    }
  } 
  return color;
}

Piece hasWon(Piece[][] board) {
  int N = board.length;
  Piece winner = Piece.Empty;

  // Check rows and columns
  for (int i = 0; i < N; i++) {

    winner = getWinner(board, i, Check.Row);
    if (winner != Piece.Empty) {
      return winner;
    }       

    winner = getWinner(board, i, Check.Column);     
    if (winner != Piece.Empty) {
      return winner;
    }

  }     

  winner = getWinner(board, -1, Check.Diagonal);
  if (winner != Piece.Empty) {
    return winner;
  }     

  // Check diagonal     
  winner = getWinner(board, -1, Check.ReverseDiagonal);
  if (winner != Piece.Empty) {
    return winner;
  } 

  return Piece.Empty;
}

Answer 1

Each time a new piece is placed in the board, you increase the appropriate row, column and (possibly ) diagonal counters by 1. You either have separate counters for the players, or use +1/-1.

Now when you check the board, you only have to check whether any of this counters equals N, which can be done in O(N) (2N+2 counters).