168. Excel Sheet Column Title

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