Description：
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 
...

Example 1:

Input: “A”
Output: 1

Example 2:

Input: “AB”
Output: 28

Example 3:

Input: “ZY”
Output: 701

题意：给定Excel工作表中显示的列标题，返回其对应的列号；

解法：列标题应当是用字母表示的27进制，每当超过27时便进一（A-1; B-2; C-3…Z-26）；例如对于AZ来说，对其再加一为BA；

Java

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