Given an integer columnNumber, return its corresponding column title as it appears in an Excel sheet.
For example:
A -> 1
B -> 2
C -> 3
...
Z -> 26
AA -> 27
AB -> 28
...
Input: columnNumber = 1
Output: "A"
Input: columnNumber = 28
Output: "AB"
Input: columnNumber = 701
Output: "ZY"
Input: columnNumber = 2147483647
Output: "FXSHRXW"