题解 | #进制转换#

import java.util.HashMap;
import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    private final static int BASE = 16;
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String nextLine = in.nextLine();
        String HexStr = nextLine.substring(2);
        StringBuffer stringBuffer = new StringBuffer(HexStr);
        StringBuffer reverse = stringBuffer.reverse();
        String reverseStr = reverse.toString();
        int sum = 0;
        HashMap hm = new HashMap();
        hm.put("0", 0);
        hm.put("1", 1);
        hm.put("2", 2);
        hm.put("3", 3);
        hm.put("4", 4);
        hm.put("5", 5);
        hm.put("6", 6);
        hm.put("7", 7);
        hm.put("8", 8);
        hm.put("9", 9);
        hm.put("A", 10);
        hm.put("B", 11);
        hm.put("C", 12);
        hm.put("D", 13);
        hm.put("E", 14);
        hm.put("F", 15);
        for (int i = 0; i < reverseStr.length(); i++) {
            char ch = reverseStr.charAt(i);
            int charInt = (int) hm.get(ch + "");
            int singleNum = charInt;
            for (int j = 0; j < i; j++) {
                singleNum *= BASE;
            }
            sum += singleNum;
        }
        System.out.println(sum);
    }
}