tic-tac-toe problem

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

//global constants
const char X = 'X';
const char O = 'O';
const char EMPTY = ' ';
const char TIE = 'T';
const char NO_ONE = 'N';



//function prototypes
void instructions();
char askYesNo(string question);
int askNumber(string question, int high, int low = 0);
char humanPiece();
char opponent(char piece);
void displayBoard(const vector<char>& board);
char winner(const vector<char>& board);
bool isLegal(const vector <char>& board, int move);
int humanMove(const vector <char>& board, char human);
int computerMove(vector<char> board, char computer);
void announceWinner(char winner, char computer, char human);

//main function

int main()
{
int move;
const int NUM_SQUARES = 9;
vector<char> board(NUM_SQUARES, EMPTY);

instructions();
char human = humanPiece();
char computer = opponent(human);
char turn = X;
displayBoard(board);

while (winner(board) == NO_ONE)
{
if (turn == human)
{
move = humanMove(board, human);
board[move] = human;
}
else
{
move = computerMove(board, computer);
board[move] = computer;
}
displayBoard(board);
turn = opponent(turn);
}
announceWinner(winner(board), computer, human);

return 0;  //Note: might put something else
}

void instructions()
{
cout<<"Welcome to the ultimate man-machine showdown:Tic-Tac-Toe.\n";
cout<<"-where human brain is pit against silicon processor.\n\n";

cout<<"Make your move known by entering a number. 0 - 8. The number\n";
cout<<"corresponds to the desired board position, as illustrated:\n\n";


cout<< "   0 | 1 | 2\n";
cout<< "   ---------\n";
cout<< "   3 | 4 | 5\n";
cout<< "   ---------\n";
cout<< "   6 | 7 | 8\n\n";

cout<<"Prepare yourself, human. The battle is about to begin.\n\n";
}

char askYesNo(string question)
{
char response;
do
{
cout<< question << " (y/n): ";
cin >> response;
} while (response != 'y' && response != 'n');

return response;
}

int askNumber(string question, int high, int low)
{
int number;
do
{
cout<< question << " (" << low << " - " << high << "): ";
cin>> number;
} while (number > high || number < low);

return number;
}


char humanPiece()
{
char go_first = askYesNo("Do you require the first move?");
if (go_first == 'y')
{
cout<< "\nThen take the first move. You will need it.\n";
return X;
}
else
{
cout<<"\nYour bravery will be your undoing... I will go first.\n";
return O;
}

}

char opponent(char piece)
{
if (piece == X)
return O;
else
return X;
}

void displayBoard(const vector<char>& board)
{
cout<<"\n\t" << board[0] << " | " << board[1] << " | " << board[2];
cout<< "\n\t" << "------";
cout<<"\n\t" << board[3] << " | " << board[4] << " | " << board[5];
cout<< "\n\t" << "------";
cout<<"\n\t" << board[6] << " | " << board[7] << " | " << board[8];
cout<<"\n\n";
}

char winner(const vector<char>& board)
{
//all possible winning rows
const int WINNING_ROWS[8][3] = { {0, 1, 2}, {3, 4, 5}, {6, 7, 8}, {0 , 3, 6}, {1, 4, 7}, {2, 5, 8}, {0, 4, 8}, {2, 4, 6}
};

const int TOTAL_ROWS = 8;

//if any  winning row has three values that are the same (and not EMPTY),
//then we have a winner
for(int row = 0; row < TOTAL_ROWS; ++row)
{
if ( (board[WINNING_ROWS[row][0]] != EMPTY) &&
(board[WINNING_ROWS[row][0]] == board[WINNING_ROWS[row][1]]) && (board[WINNING_ROWS[row][1]] == board[WINNING_ROWS[row][2]]) )
{
return board[WINNING_ROWS[row][0]];
}

}

//since nobody has won, check for a tie (no empty sqaures left)

if (count(board.begin(), board.end(), EMPTY) == 0)
return TIE;

//since nobody has won and it isn't a tie, the game ain't over
return NO_ONE;
}

inline bool isLegal(int move, const vector<char>& board)
{
return (board[move] == EMPTY);
}

int humanMove(const vector<char>& board, char human)
{
int move = askNumber("Where will you move?", (board.size() - 1));
while (!isLegal(move, board))
{
cout<< "\nThat square is already occupied, foolish human.\n";
move = askNumber("Where will you move?", (board.size() -1));
}
cout<< "Fine...\n";
return move;
}

int computerMove(vector<char> board, char computer)
{
cout<<"I shall take square number ";

//if computer can win on next move, make that move
for(int move = 0; move < board.size(); ++move)
{
if (isLegal(move, board))
{
board[move] = computer;
if (winner(board) == computer)
{
cout<< move << endl;
return move;
}
//done checking this move, undo it
board[move] = EMPTY;
}
}


//if human can win on next move, block that move
char human = opponent(computer);

for(int move = 0; move < board.size(); ++move)
{
if (isLegal(move, board))
{
board[move] = human;
if (winner(board) == human)
{
cout<< move << endl;
return move;
}
//done checking this move,undo it
board[move] = EMPTY;
}
}

//the best moves to make, in order
const int BEST_MOVES[] = {4, 0, 2, 6, 8, 1, 3, 5, 7};
//since no one can win on next move, pick best open square
for (int i = 0; i < board.size(); ++i)
{
int move = BEST_MOVES[i];
if (isLegal(move, board))
{
cout<< move << endl;
return move;
}
}
}
void announceWinner(char winner, char computer, char human)
{
if (winner == computer)
{
cout<< winner << "'s won!\n";
cout<< "As I predicted human, I am triumphant once more -- proof\n";
cout<< " that computers are superior to humans in all regards.\n";
}
else if (winner == human)
{
cout<< winner << " 's won!\n";
cout<< "No, no! It cannot be! Somehow you tricked me, human.\n";
cout<< "But never again I, the computer, so swear it!\n";
}

else
{
cout<< "It's a tie\n";
cout<< "You were most lucky human, and somehow managed to tie me.\n";
cout<< "Celebrate... for this is the best you will ever achieve.\n";
}
}