A -> 1
B -> 2
C -> 3
   ...
Z -> 26
AA -> 27
AB -> 28

 1 class Solution {
 2 public:
 3     int titleToNumber(string s) {
 4         int num = 0;
 5         for (int i = 0; i < s.length(); i++)
 6         {
 7             char ch = s[i];
 8             num = num * 26 + (ch - ‘A‘ + 1);
 9         }
10         
11         return num;
12     }
13 };