LeetCode 171. Excel Sheet Column Number

class Solution{
public:
int pow_(int n){
int res = 1;
for(int i = 0; i < n; i++)
res *= 26;
return res;
}
int titleToNumber(string s){
int number = 0;
int len = s.length();
for(int i = len - 1; i >= 0; i--){
number += (s[i] - 'A'+ 1) * pow_(len - i - 1);
}
return number;
}
};