Leetcode 171 Excel Sheet Column Number python

题目：

Given a column title as appear in an Excel sheet, return its corresponding column number.

For example:

    A -> 1
    B -> 2
    C -> 3
    ...
    Z -> 26
    AA -> 27
    AB -> 28 
 
26进制

 class Solution(object):
     def titleToNumber(self, s):
         length = len(s)
         if length == 0: return 0
         ans = 0
         for i in range(length):
             ans = ans * 26 + ord(s[i]) - ord('A') + 1
         return ans