166. Fraction to Recurring Decimal

首先这题要考虑，若被除数为0，显然返回0，如果两个数一正一负，显然是不好处理的，因此要先把除法结果的正负号记下来。然后对被除数和除数都做取绝对值的处理。
为了处理循环小数，用一个哈希表记录下每一步下来的余数，如果这一步的余数和之前的重复说明陷入循环，就需要找到之前余数出现的位置前，对他进行加括号处理，具体代码如下：

class Solution:
    # @return a string
    def fractionToDecimal(self, numerator, denominator):
        ## denominator can be 0 but do not need to consider here
        if numerator == 0:
            return '0'
        neg = False
        if numerator > 0 and denominator < 0 or numerator < 0 and denominator > 0:
            neg = True
        if numerator % denominator == 0:
            return str(numerator / denominator)
        numerator = abs(numerator)
        denominator = abs(denominator)
        table = {}
        res = ""
        res += str(numerator / denominator)
        res += '.'
        numerator %= denominator
        i = len(res)
        while numerator:
            if numerator not in table:
                table[numerator] = i
            else:
                i = table[numerator]
                res = res[:i] + '(' + res[i:] + ')'
                if neg:
                    return '-' + res
                else:
                    return res
            numerator = numerator * 10
            res += str(numerator/denominator)
            numerator %= denominator
            i+=1
        if neg:
            return '-' + res
        return res