166. Fraction to Recurring Decimal

    public String fractionToDecimal(int numerator, int denominator) {
     
        if(numerator == 0)
            return "0";
        
        StringBuilder res = new StringBuilder();
        
        // "+" or "-", ^ :  1 ^ 0 = 1, 1^1 = 0^0 = 0
        res.append(((numerator > 0) ^ (denominator > 0)) ? "-" : "");
        long num = Math.abs((long)numerator);
        long den = Math.abs((long)denominator);    // 防止  -2147483648 溢出

        // Integer part
        res.append(num / den);
        num %= den;
        if(num == 0)
            return res.toString();
        
        // fractional part
        res.append(".");
        HashMap<Long, Integer> map = new HashMap<>();
        map.put(num, res.length());
        while(num != 0) {
            num *= 10;    // *
            res.append(num / den);
            num %= den;
            // 判断是否为无限循环小数
            if(map.containsKey(num)) {
                int index = map.get(num);
                res.insert(index, "(");
                res.append(")");
                break;
            }
            else {
                map.put(num, res.length());
            }
        }
        return res.toString();                                                
    }