Create a Tic tac toe game using c++

The following program is the source code of a game called tic tac toe. The program is implemented using c++.

Code :

#include<iostream>
#include<windows.h>
using namespace std;
char a[9]={'1','2','3','4','5','6','7','8','9'};
char c1,c2;
void draw_board()
{
    cout<<"nn";
    cout<<"   "<<a[0]<<"   |   "<<a[1]<<"   |    "<<a[2]<<"   ";
    cout<<endl<<"       |       |       ";
    cout<<"n-------|-------|-------";
    cout<<endl<<"   "<<a[3]<<"   |   "<<a[4]<<"   |    "<<a[5]<<"   ";
    cout<<endl<<"       |       |       ";
    cout<<"n-------|-------|-------";
    cout<<endl<<"   "<<a[6]<<"   |   "<<a[7]<<"   |    "<<a[8]<<"   ";
    cout<<endl<<"       |       |       ";

}
int check_res()
{
    if(a[0]==a[1] && a[1]==a[2])
    {
        if(a[0]==c1)
            return 1;
        else
            return 2;
    }

    else if(a[0]==a[3] && a[3]==a[6])
    {
        if(a[0]==c1)
            return 1;
        else
            return 2;
    }
    else if(a[0]==a[4] && a[4]==a[8])
    {
        if(a[0]==c1)
            return 1;
        else
            return 2;
    }
    else if(a[1]==a[4] && a[4]==a[7])
    {
        if(a[1]==c1)
            return 1;
        else
            return 2;
    }
    else if(a[2]==a[4] && a[4]==a[6])
    {
        if(a[2]==c1)
            return 1;
        else
            return 2;
    }
    else if(a[2]==a[5] && a[5]==a[8])
   {
        if(a[2]==c1)
            return 1;
        else
            return 2;
    }
    else if(a[3]==a[4] && a[4]==a[5])
    {
        if(a[3]==c1)
            return 1;
        else
            return 2;
    }
    else if(a[6]==a[7] && a[7]==a[8])
    {
        if(a[6]==c1)
            return 1;
        else
            return 2;
    }
    else
        return 0;

}
void play_game()
{

    int c,r;
    int p;
    cout<<"nEnter your choice : n1.Xt 2.On";
    cin>>c;
    if(c==1)
    {
        c1='X';
        c2='O';
    }
    else if(c==2)
    {
        c1='O';
        c2='X';
    }
    else
    {
        cout<<"nInvalid choice...";

    }
    c=0;
    while(c<10)
    {
        A:
        cout<<"n==>Player 1 turn :";
        cout<<"nEnter your choice : ";
        cin>>p;
        if(isdigit(a[p-1]))
        {
            c++;
            a[p-1]=c1;
            system("cls");
            draw_board();
            r=check_res();
            if(r!=0)
                break;

        }
        else
        {
            cout<<"nAlready filled up";
            goto A;
        }
        B:
        cout<<"n==>Player 2 turn :";
        cout<<"nEnter your choice : ";
        cin>>p;
        if(isdigit(a[p-1]))
        {
            a[p-1]=c2;
            system("cls");
            draw_board();
            r=check_res();
            c++;
            if(r!=0)
                break;

        }
        else
        {
            cout<<"nAlready filled up";
            goto B;
        }
    }
    if(r!=0)
            cout<<"nPlayer "<<r<<" wins the match";
    else
        cout<<"nDraw";

}

int main()
{
    draw_board();
    play_game();
    return 0;
}