171.EXcel表列序号

方法：进制转换

class Solution {
    public int titleToNumber(String columnTitle) {
        //进制转换
        int number = 0;
        int mul = 1;
        for(int i = columnTitle.length()-1;i>=0;i--){
            int k = columnTitle.charAt(i) - 'A' + 1;
            number += mul * k;
            mul *= 26;
        }
        return number;
    }
}