题目描述：
Related to question Excel Sheet Column Title
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
本题可以转化为进制转换问题，
LeetCode Excel Sheet Column Number
，电脑资料
《LeetCode Excel Sheet Column Number》(https://www.unjs.com)。思路：和2进制转10进制有点类似，对于AAA，可以理解为26进制的转换：1*26 + 1*26^2 + 1*26^3
实现代码：
public class Solution { public int TitleToNumber(string s) { var sum = 0; for(var i = s.Length - 1; i >= 0; i--){ var d = s.Length - i; var n = Num(s[i]); sum += (int)Math.Pow(26, d-1) * n; } return sum; } private int Num(char c){ return (int)c - 64;}}