题目描述：
Given a non-zero 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
题目大意：
给定一个正整数，返回其在Excel表格中对应的列标题 ...
Given a non-zero 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
给定一个正整数，返回其在Excel表格中对应的列标题 ...