题解 | #进制转换#

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String mat = sc.next();
        int res = 0;
        int tmp = 0;
	  // 先处理最低位，从后往前读，前两位是0X，不读
        for(int i = mat.length() - 1; i > 1; i--) {
            char ch = mat.charAt(i);
		  // Java 中字符用Unicode存储，0-9是1-10，A-Z是65-90，a-z是97-122，把ch转换成int
            if(ch <= '9') {
                ch -= '0';
                tmp = ch;
            } else if(ch <= 'Z') {
                ch -= 'A';
                tmp = 10 + ch;
            } else if(ch <= 'z') {
                ch -= 'a';
                tmp = 10 + ch;
            }
		  // 每一位的位权为16的n次方，n是长度减2再减索引加1
            res += tmp * Math.pow(16, mat.length() - 2 - i + 1);
        }
        System.out.println(res);
    }
}