171. Excel表列序号

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

输入: "A"
输出: 1

输入: "AB"
输出: 28

输入: "ZY"
输出: 701

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