Q171 Excel Sheet Column Number

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

class Solution(object):
    def titleToNumber(self, s):
        """
        :type s: str
        :rtype: int
        """
        lens = len(s)
        if lens == 0:
            return 0
        ans, p, i = 0, 0, lens - 1
        while i >= 0:
            ans += (ord(s[i]) - ord('A') + 1) * pow(26, p)
            p += 1; i -= 1
        return ans

a = 'BA'
b = 'ZZ'
c = Solution()
print(c.titleToNumber(a))  # 53
print(c.titleToNumber(b))  # 702