171. Excel Sheet Column Number

原题链接：https://leetcode.com/problems/excel-sheet-column-number/description/
这题没啥技术含量，直接就是实现：

/**
 * Created by clearbug on 2018/2/26.
 */
public class Solution {

    public static void main(String[] args) {
        Solution s = new Solution();
        System.out.println(s.titleToNumber("A"));
        System.out.println(s.titleToNumber("B"));
        System.out.println(s.titleToNumber("Z"));

        System.out.println(s.titleToNumber("AA"));
        System.out.println(s.titleToNumber("AB"));
        System.out.println(s.titleToNumber("AZ"));
    }

    public int titleToNumber(String s) {
        int res = 0;
        for (int i = 0; i < s.length(); i++) {
            res += titleToNumberHelper(s.charAt(i)) * Math.pow(26, s.length() - 1 - i);
        }
        return res;
    }

    private int titleToNumberHelper(char c) {
        return c - 'A' + 1;
    }
}