class Solution {
    public int titleToNumber(String columnTitle) {
        //定义一个要返回的总数
        int number = 0;
        //定义一个相乘系数，最右边的乘以1，往前进一位再乘以26
        int multiple = 1;
        for(int i = columnTitle.length() -1; i >= 0; i--){
            int k = columnTitle.charAt(i) - 'A' + 1;
            number += k * multiple;
            multiple *= 26;
        }
        return number;
    }
}