大数面试题

• 16进制转10进制
  写出一个程序，接受一个十六进制的数值字符串，输出该数值的十进制字符串。（多组同时输入 ）

        char2Digit.put('0', "0");
        char2Digit.put('1', "1");
        char2Digit.put('2', "2");
        char2Digit.put('3', "3");
        char2Digit.put('4', "4");
        char2Digit.put('5', "5");
        char2Digit.put('6', "6");
        char2Digit.put('7', "7");
        char2Digit.put('8', "8");
        char2Digit.put('9', "9");
        char2Digit.put('A', "10");
        char2Digit.put('B', "11");
        char2Digit.put('C', "12");
        char2Digit.put('D', "13");
        char2Digit.put('E', "14");
        char2Digit.put('F', "15");

        Scanner in = new Scanner(System.in);
        while (in.hasNextLine()){
            String hexString = in.nextLine();
            hexString = hexString.substring(2);
            BigInteger num = new BigInteger("0");
//            Integer num = Integer.valueOf(hexString, 16);
            for (int i = 0; i < hexString.length(); i++) {
                char ch = hexString.charAt(i);
                num = num.multiply(new BigInteger("16"));
                num = num.add(new BigInteger(char2Digit.get(ch)));
            }
            System.out.println(num.toString());
        }