Problem

a

Solution

s

public static String convertBase(String numAsString, int b1, int b2) {
    boolean isNegative = numAsString.startsWith("-");
    int numAsInt = 0;
    for (int i = (isNegative ? 1 : 0); i < numAsString.length(); ++i) {
      numAsInt *= b1;
      numAsInt += Character.isDigit(numAsString.charAt(i))
                      ? numAsString.charAt(i) - '0'
                      : numAsString.charAt(i) - 'A' + 10;
    }
    return (isNegative ? "-" : "")
        + (numAsInt == 0 ? "0" : constructFromBase(numAsInt, b2));
}

private static String constructFromBase(int numAsInt, int base) {
    return numAsInt == 0
        ? ""
        : constructFromBase(numAsInt / base, base)
          + (char)(numAsInt % base >= 10 ? 'A' + numAsInt % base - 10
                                         : '0' + numAsInt % base);
}

s2

References