Given a positive integer, return its corresponding column title as appear in an Excel sheet.
For example:
1 -> A
2 -> B
3 -> C
...
26 -> Z
27 -> AA
28 -> AB
...
Example 1:
Input: 1 Output: "A"
Example 2:
Input: 28 Output: "AB"
Example 3:
Input: 701 Output: "ZY"