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

class Solution(object):    def convertToTitle(self, n):        """        :type n: int        :rtype: str        """        result = ''        while n:            result += chr((n-1)%26+65)            n = (n-1)/26        return result[-1::-1] #注意逆序