171. Excel Sheet Column Number

题目描述：

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 

思路一：

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

思路二：
class Solution {
public int titleToNumber(String s) {
int res = 0;
for (int i = s.length() - 1, j = 0; i >= 0 && j <s.length(); i--, j++)
res += Math.pow(26, i) * (s.charAt(j) - 'A' + 1);
return res;
}
}