进制转换，java做

StringBuffer 的方法中setCharAt(int,char)可以改变其某位的字符

import java.io.*;import java.util.*;import java.math.*;public class Main {    public static void main(String[] args)    {        Scanner cin = new Scanner(System.in);        while (cin.hasNext())        {            String st = cin.next();            int a = cin.nextInt();            int b = cin.nextInt();            BigInteger bigA = new BigInteger(st, a);            StringBuffer ans = new StringBuffer(bigA.toString(b));            if (ans.length() > 7)            {                System.out.println("  ERROR");                continue;            }            for (int i = 0; i < ans.length(); i++)                if (ans.charAt(i) <= 'z' && ans.charAt(i) >= 'a')                    ans.setCharAt(i, (char)('A' - 'a' + ans.charAt(i)));            int len = ans.length();            for (int i = len; i < 7; i++)                ans.insert(0,  ' ');            System.out.println(ans.toString());        }    }}