Related to question 

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

1 class Solution { 2 public: 3     int titleToNumber(string s) { 4         auto it = s.end()-1; 5         char c; 6         int num =0; 7         int i=1; 8         while(it>=s.begin()) { 9             c = *it;10             num += (c-'A'+1)*i;11             i *= 26;12             --it;13         }14         return num;15     }16 };

模仿整数的操作即可。