Excel Column Number

Given a column title as appears in an Excel sheet, return its corresponding column number.

Example:

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

Time: O(n)
Space: O(1)

public class Solution {
    public int titleToNumber(String A) {
        int size = 26;
        int n = A.length() - 1;
        int number = 0;
        for (int i = n; i >= 0; i --) {
            char c = A.charAt(i);
            number += (c - 'A' + 1) * (int) Math.pow(size, n - i);
        }
        return number;
    }
}