171. Excel Sheet Column Number

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


可转化为求26进制的问题，有迭代和递归两种方式

class Solution(object):
    def titleToNumber(self, s):
        """
        :type s: str
        :rtype: int
        """
        n=len(s)

        #迭代方法
        result=0
        for i in range(n):            
             result=result*26+ord(s[i])-64        
        return result

       #递归方法
        if n == 1:
            return ord(s)-64
        else:
            return ord(s[-1])-64 + 26*self.titleToNumber(s[:-1])