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 

思路：

26進位制轉換為10進位制。

Code：

class Solution {
public:
    int titleToNumber(string s) {
        int ans=0;
        int len=s.size();
        for(int i=len;i>0;i--){
            ans+=(s[i-1]-'A'+1)*pow(26,len-i);        
        }
        return ans;
    }
};