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 

    int titleToNumber(string s) {
        vector<int> a(s.size());
        int i,sum=0;
        for(i=0;i<s.size();i++)
            a[i]=int(s[i])-int('A')+1;
        for(i=s.size()-1;i>=0;i--)
        {
            sum+=pow(26,i)*a[s.size()-i-1];
        }
        return sum;
    }