k进制转十进制

import java.util.Scanner;

public class Main {
    public static int cal(char c) {
        if (c >= 'A')//字符是字母
            return 10 + c - 'A';
        return c - '0';//字符是数字
    }
    //将给定字符串 s 转化为十进制数, k 是目标进制
    public static int change(int k,String s) {
        int ans = 0;//初始值
        //正序模板(从高到低)
        for (int i = 0;i < s.length();i ++)
            ans = ans * k + cal(s.charAt(i));   
        return ans;
    }
    public static void main(String[] args) {
         Scanner sc = new Scanner(System.in); 
         String s = sc.next();
         System.out.print(change(16,s));
    }
}

十进制转k进制

import java.util.Scanner;

public class Main {
    public static String change(int x,int k) {
        StringBuilder ans = new StringBuilder();
        while (x != 0) {
             int t = x % k;//余数作为当前位的数
             if (t <= 9) 
                 ans.append((char)('0'+ t));
             else 
                 ans.append((char)('A' + t - 10));
             x /= k;//更新x,商用于下一次迭代
        }
        return ans.reverse().toString();
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int s = sc.nextInt();
        System.out.print(change(s,16));
    }
}