Fraction to Recurring Decimal.java

/*Given two integers representing the numerator and denominator of a fraction, return the fraction in string format.
If the fractional part is repeating, enclose the repeating part in parentheses.
For example,
Given numerator = 1, denominator = 2, return "0.5".
Given numerator = 2, denominator = 1, return "2".
Given numerator = 2, denominator = 3, return "0.(6)".*/

public class Solution {

    public String fractionToDecimal(int numerator, int denominator) {
        if (denominator == 0)
            return "NaN";
        if (numerator == 0)
            return "0";
        StringBuilder result = new StringBuilder();
        Long n = new Long(numerator);
        Long d = new Long(denominator);
        // negative or positive
        if (n*d < 0)
            result.append("-");
        n = Math.abs(n);
        d = Math.abs(d);
        result.append(Long.toString(n / d));
        // result is integer or float
        if (n % d == 0)
            return result.toString();
        else
            result.append(".");
        // deal with the float part
        // key is the remainder, value is the start positon of possible repeat numbers
        HashMap<Long, Integer> map = new HashMap<Long, Integer>();
        Long r = n % d;
        while (r > 0) {
            if (map.containsKey(r)) {
                result.insert(map.get(r), "(");
                result.append(")");
                break;
            }
            map.put(r, result.length());
            r *= 10;
            result.append(Long.toString(r / d));
            r %= d;
        }
        return result.toString();
    }
}