public static bool Validate(int row, int col, int? value, SudokuGrid grid, out string message)
{
if (value == null)
{
message = null;
return true;
}
if (grid.IsValueAvailable(row, col, value.Value))
{
message = null;
return true;
}
else
{
message = "Invalid input. Duplicate value present.";
return false;
}
}