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 

class Solution {
public:
    int titleToNumber(string s) {
        if(s.size() <= 0) return 0;
        int col = s[0] - 'A' + 1;
        for(int i=1;i<s.size();i++){
            col = col * 26 + s[i] - 'A' + 1;
        }
        return col;
    }
};