需要注意的是是：在 Map 中放的是被除数，而不是计算出来的位数digit。这样更方便处理！
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)"
link:{sourcedir}/_0166_FractionToRecurringDecimal.java[role=include]