Description:

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.

Example 1:

Input: numerator = 1, denominator = 2
Output: "0.5"

Example 2:

Input: numerator = 2, denominator = 1
Output: "2"

Example 3:

Input: numerator = 2, denominator = 3
Output: "0.(6)"

Solution:

NOTE: 各种edge cases，相当恶心。比如Python str(1/2147483648)结果是“0”，因为根本不知道要求是什么（输入分子分母的范围），所以也无法确定应该使用的方法；比如"0.0"不行非要输出"0"。唯一有意思的是对无限循环位数的判断，在slot函数里。digits这个变量表示小数点以后显示多少位，包括不循环和循环的部分。

class Solution:
    def fractionToDecimal(self, numerator: int, denominator: int) -> str:
        if numerator/denominator == numerator//denominator: # 22/2 => 11 not 11.0
            return str(numerator//denominator) # 0/-5 => 0 not 0.0
        
        gcd = math.gcd(numerator,denominator) # 50/8 => 6.25 not 6.250
        numerator //= gcd
        denominator //= gcd
        
        if denominator == 1: # 2.0 => 2
            return str(numerator)
        
        sign = int(numerator*denominator < 0) # -7/12 => -7//12 和 -7%12在Python里都是错的结果
        numerator = abs(numerator)
        denominator = abs(denominator)
        
        def slot(denominator):
            i = 10
            cnt = 1
            while (i-1)%denominator != 0:
                i *= 10
                cnt += 1
            return cnt
            
        def helper(nume,deno):
            two = 0
            while deno%2 == 0:
                deno //= 2
                two += 1
            five = 0
            while deno%5 == 0:
                deno //= 5
                five += 1
            
            nume *= pow(10,max(two,five))
            nume //= pow(2,two)*pow(5,five)
            
            digits = max(two,five)
            if deno != 1:
                if digits == 0:
                    ret = ".("
                else:
                    ret = "." + str(nume//deno).zfill(digits) + "("
                nume %= deno
                cache = ""
                for i in range(slot(deno)):
                    nume *= 10
                    cache += str(nume//deno)
                    nume = nume%deno
                if int(cache) == 0:
                    return ""
                else:
                    return ret + cache + ")"
            else:
                return "." + str(nume).zfill(digits)
            
        return str(sign*"-") + str(numerator//denominator) + helper(numerator%denominator,denominator)

Runtime: 32 ms, faster than 92.16% of Python3 online submissions for Fraction to Recurring Decimal.
Memory Usage: 13.8 MB, less than 5.95% of Python3 online submissions for Fraction to Recurring Decimal.