C++ Tic Tac Toe Game

#include <iostream>
using namespace std;

char square[10] = {'o','1','2','3','4','5','6','7','8','9'};

int checkwin();
void board();

int main()
{
 int player = 1,i,choice;

 char mark;
 do
 {
  board();
  player=(player%2)?1:2;

  cout << "Player " << player << ", enter a number:  ";
  cin >> choice;

  mark=(player == 1) ? 'X' : 'O';

  if (choice == 1 && square[1] == '1')

   square[1] = mark;
  else if (choice == 2 && square[2] == '2')

   square[2] = mark;
  else if (choice == 3 && square[3] == '3')

   square[3] = mark;
  else if (choice == 4 && square[4] == '4')

   square[4] = mark;
  else if (choice == 5 && square[5] == '5')

   square[5] = mark;
  else if (choice == 6 && square[6] == '6')

   square[6] = mark;
  else if (choice == 7 && square[7] == '7')

   square[7] = mark;
  else if (choice == 8 && square[8] == '8')

   square[8] = mark;
  else if (choice == 9 && square[9] == '9')

   square[9] = mark;
  else
  {
   cout<<"Invalid move ";

   player--;
   cin.ignore();
   cin.get();
  }
  i=checkwin();

  player++;
 }while(i==-1);
 board();
 if(i==1)

  cout<<"==>\aPlayer "<<--player<<" win ";
 else
  cout<<"==>\aGame draw";

 cin.ignore();
 cin.get();
 return 0;
}

/*********************************************

 FUNCTION TO RETURN GAME STATUS
 1 FOR GAME IS OVER WITH RESULT
 -1 FOR GAME IS IN PROGRESS
 O GAME IS OVER AND NO RESULT
**********************************************/

int checkwin()
{
 if (square[1] == square[2] && square[2] == square[3])

  return 1;
 else if (square[4] == square[5] && square[5] == square[6])

  return 1;
 else if (square[7] == square[8] && square[8] == square[9])

  return 1;
 else if (square[1] == square[4] && square[4] == square[7])

  return 1;
 else if (square[2] == square[5] && square[5] == square[8])

  return 1;
 else if (square[3] == square[6] && square[6] == square[9])

  return 1;
 else if (square[1] == square[5] && square[5] == square[9])

  return 1;
 else if (square[3] == square[5] && square[5] == square[7])

  return 1;
 else if (square[1] != '1' && square[2] != '2' && square[3] != '3' 
                    && square[4] != '4' && square[5] != '5' && square[6] != '6' 
                  && square[7] != '7' && square[8] != '8' && square[9] != '9')

  return 0;
 else
  return -1;
}


/*******************************************************************
     FUNCTION TO DRAW BOARD OF TIC TAC TOE WITH PLAYERS MARK
********************************************************************/


void board()
{
 system("cls");
 cout << "\n\n\tTic Tac Toe\n\n";

 cout << "Player 1 (X)  -  Player 2 (O)" << endl << endl;
 cout << endl;

 cout << "     |     |     " << endl;
 cout << "  " << square[1] << "  |  " << square[2] << "  |  " << square[3] << endl;

 cout << "_____|_____|_____" << endl;
 cout << "     |     |     " << endl;

 cout << "  " << square[4] << "  |  " << square[5] << "  |  " << square[6] << endl;

 cout << "_____|_____|_____" << endl;
 cout << "     |     |     " << endl;

 cout << "  " << square[7] << "  |  " << square[8] << "  |  " << square[9] << endl;

 cout << "     |     |     " << endl << endl;
}

/*******************************************************************
    END OF PROJECT
********************************************************************/