Spreadsheet columns to number #124
Description
This problem was asked by Dropbox.
Spreadsheets often use this alphabetical encoding for its columns: "A", "B", "C", ..., "AA", "AB", ..., "ZZ", "AAA", "AAB", ....
Given a column number, return its alphabetical column id. For example, given 1, return "A". Given 27, return "AA".