Dear God What Do We Have Here :)

#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;
}


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;
}