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)".

这题实现起来还是比较麻烦的，主要还要考虑负数的输入情况，特别是-2147483648这样的整数型的边界值。最简单的方法就是先把所有的输入全部转换为长整型的正数。

另外，由于要记录小数里循环的部分，所以还需要用到一个Map去记录小数点的每位和对应的余数值。如果碰到的同样的余数值，那么说明遇到了开始循环的部分了。

	public String fractionToDecimal(int numerator, int denominator) {
		long longNumerator = Math.abs((long) numerator);
		long longDenominator = Math.abs((long) denominator);

		StringBuilder sb = new StringBuilder();
		if ((long) numerator * denominator < 0) sb.append("-");
		sb.append(longNumerator / longDenominator);

		long remainder = longNumerator % longDenominator;
		if (remainder == 0) return sb.toString();
		sb.append(".");

		StringBuilder fracSb = new StringBuilder();
		Map<Long, Integer> map = new HashMap<Long, Integer>();
		int index = 0;
		while (remainder != 0) {
			remainder *= 10;
			long nextRemainder = remainder % longDenominator;
			// If repeated part occurs.
			if (map.containsKey(remainder)) {
				fracSb.insert(map.get(remainder), "(");
				fracSb.append(")");
				break;
			}
			
			map.put(remainder, index++);
			fracSb.append(remainder / longDenominator);
			remainder = nextRemainder;
		}

		sb.append(fracSb);
		return sb.toString();
	}

[LeetCode] Fraction to Recurring Decimal