A -> 1

    B -> 2

    C -> 3

    ...

    Z -> 26

    AA -> 27

    AB -> 28 

 class Solution {

 public:

     int titleToNumber(string s) {

         map<char, int> map;

         int num = ;

         int result = ;

         for (char i = 'A'; i<='Z'; i++){

             map[i] = num;

             num++;

         }

         int j = ;

         for(auto c = s.crbegin(); c<s.crend(); c++){

             result = map[*c]*pow(, j) + result;

             j++;

         }

         return result;

     }

 };