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

public class Solution {
    public int titleToNumber(String s) {
        int ret = 0;
        for (int i=0;i<s.length();i++)
            ret = ret*26+(s.charAt(i)-'A'+1);
        return ret;
    }
}

class Solution(object):
    def titleToNumber(self, s):
        """
        :type s: str
        :rtype: int
        """
        col = dict()
        for i in range(26):
            col[str(unichr(65+i))] = i+1
        ret = 0
        for i in range(len(s)-1,-1,-1):
            ret += col[s[i]]*(26 ** (len(s)-i-1))
        return ret