166. 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)”.
Credits:
Special thanks to @Shangrila for adding this problem and creating all test cases.

class Solution {
    public String fractionToDecimal(int numerator, int denominator) {
        if (numerator == 0) return "0";  
        if (denominator == 0) return "";  

        String ans = "";  

        //如果结果为负数  
        if ((numerator < 0) ^ (denominator < 0)) {  
            ans += "-";  
        }  

        //下面要把两个数都转为正数，为避免溢出，int转为long  
        long num = numerator, den = denominator;  
        num = Math.abs(num);  
        den = Math.abs(den);  

        //结果的整数部分  
        long res = num / den;  
        ans += String.valueOf(res);  

        //如果能够整除，返回结果  
        long rem = (num % den) * 10;  
        if (rem == 0) return ans;  

        //结果的小数部分  
        HashMap<Long, Integer> map = new HashMap<Long, Integer>();  
        ans += ".";  
        while (rem != 0) {  
            //如果前面已经出现过该余数，那么将会开始循环  
            if (map.containsKey(rem)) {  
                int beg = map.get(rem); //循环体开始的位置  
                String part1 = ans.substring(0, beg);  
                String part2 = ans.substring(beg, ans.length());  
                ans = part1 + "(" + part2 + ")";  
                return ans;  
            }  

            //继续往下除  
            map.put(rem, ans.length());  
            res = rem / den;  
            ans += String.valueOf(res);  
            rem = (rem % den) * 10;  
        }  

        return