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
...
Example 1:
Input: “A”
Output: 1
Example 2:
Input: “AB”
Output: 28
Example 3:
Input: “ZY”
Output: 701
思路： 以26为周期 先看输入字符串的长度 其实就是26进制转化为10进制的思想（参考网上https://blog.csdn.net/NXHYD/article/details/71713837）
class Solution(object):
def titleToNumber(self, s):
"""
:type s: str
:rtype: int
"""
Sum = 0
length = len(s)
if length == 0:
return 0
for i in range(0, length):
# print(i)
# print(length-i-1)
Sum += pow(26, length-i-1) * (ord(s[i])-64)
return Sum
if __name__ == "__main__":
c = "AB"
print(Solution().titleToNumber(c))
注：
ord（）函数就是用来返回单个字符的ascii值（0-255）或者unicode数值 采用ord()-64可以得到相应整数 pow函数的使用
