Excel column title

Convert a positive integer column number to its Excel/spreadsheet column title (1 -> 'A', 26 -> 'Z', 27 -> 'AA', 28 -> 'AB', 702 -> 'ZZ', 703 -> 'AAA'). This is bijective base-26 (there is no zero digit). Constraints: 1 <= n <= 2^31 - 1. Return the title string.