171. Excel Sheet Column Number

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 


C++（6ms）：

 1 class Solution {
 2 public:
 3     int titleToNumber(string s) {
 4         int ans = 0 ;
 5         int k = 0 ;
 6         for (int i = s.size()-1; i >= 0;i-- ){
 7             ans += (s[i]-'A'+1)*pow(26,k++);
 8         }
 9         return ans ;
10     }
11 };

C++（3ms）：

 1 class Solution {
 2 public:
 3     int titleToNumber(string s) {
 4         int ans = 0 ;
 5         int k = 0 ;
 6         for (int i = 0; i < s.size();i++ ){
 7             ans = ans * 26 + (s[i]-'A'+1);
 8         }
 9         return ans ;
10     }
11 };