168. Excel Sheet Column Title

感觉自己有点愚蠢做的这一题，其实就是把它变成0-25之间就清楚了
A 1 AA 26+ 1 BA 2×26+ 1 ... ZA 26×26+ 1 AAA 1×26²+1×26+ 1
B 2 AB 26+ 2 BB 2×26+ 2 ... ZB 26×26+ 2 AAB 1×26²+1×26+ 2
. . .. ..... .. ....... ... .. ........ ... .............
. . .. ..... .. ....... ... .. ........ ... .............
. . .. ..... .. ....... ... .. ........ ... .............
Z 26 AZ 26+26 BZ 2×26+26 ... ZZ 26×26+26 AAZ 1×26²+1×26+26

now we can see that ABCD＝A×26³＋B×26²＋C×26¹＋D＝1×26³＋2×26²＋3×26¹＋4

But how to get the column title from the number? We can't simply use the n%26 method because:

ZZZZ＝Z×26³＋Z×26²＋Z×26¹＋Z＝26×26³＋26×26²＋26×26¹＋26

class Solution(object):
    def convertToTitle(self, n):
        """
        :type n: int
        :rtype: str
        """
        alpha = ['A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z']
        stack = []
        while n:
            stack.append(alpha[(n-1) % 26])
            n = (n-1) / 26
        stack.reverse()
        return ''.join(stack)