You are writing to memory you do not own:
int board[2][50]; //make an array with 3 columns (wrong)
//(actually makes an array with only two 'columns')
...
for (i=0; i<num3+1; i++)
board[2][i] = 'O';
^
Change this line:
int board[2][50]; //array with 2 columns (legal indices [0-1][0-49])
^
To:
int board[3][50]; //array with 3 columns (legal indices [0-2][0-49])
^
When creating an array, the value used to initialize: [3] indicates array size.
However, when accessing existing array elements, index values are zero based.
For an array created: int board[3][50];
Legal indices are board[0][0]...board[2][49]
EDIT To address bad output comment and initialization comment
add an additional "\n" for formatting output:
Change:
...
for (k=0; k<50;k++) {
printf("%d",board[j][k]);
}
}
...
To:
...
for (k=0; k<50;k++) {
printf("%d",board[j][k]);
}
printf("\n");//at the end of every row, print a new line
}
...
Initialize board variable:
int board[3][50] = {0};//initialize all elements to zero