166. Fraction to Recurring Decimal

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)".

题目含义：计算两个数的除法，出现无限循环位置时，用括号括起来

    public String fractionToDecimal(int numerator, int denominator) {
        if (denominator ==0) return "";
        StringBuilder result = new StringBuilder();
        String sign = (numerator < 0 == denominator < 0 || numerator == 0) ? "" : "-";
        long num = Math.abs((long) numerator);
        long den = Math.abs((long) denominator);
        result.append(sign);
        result.append(num / den);
        long remainder = num % den;
        if (remainder == 0)
            return result.toString();
        result.append(".");
        HashMap<Long, Integer> hashMap = new HashMap<Long, Integer>();
        while (!hashMap.containsKey(remainder)) {
            hashMap.put(remainder, result.length());
            result.append(10 * remainder / den);
            remainder = 10 * remainder % den;
        }
        int index = hashMap.get(remainder);
        result.insert(index, "(");
        result.append(")");
        return result.toString().replace("(0)", "");        
    }