given a positive integer, return its corresponding column title as appear in an excel sheet.
for example:
because <code>any_pos_int mod 26</code> should return a number in the interval <code>[0, 25]</code>, but what we want is a number in the interval <code>[1, 26]</code>. thus we have to shift the digit leftward by <code>1</code> which
means<code>--n</code>.
