[LeetCode] 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)".

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();
}