gridmaniac

for (int k = size; k >= -size; k--)
{
  for (int j = 0; j < size - k; j++)
  {
    int i = k + j;
    if (i < size && j < size && i >= 0 && j >= 0)
    {
      diagonal += arr2[j, i] + " ";
    }
  }

  Console.WriteLine(diagonal);
  diagonal = "";
}