Fraction to Recurring Decimal

题目
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.

答案

class Solution {
    public String fractionToDecimal(int n, int d) {
        Map<Long, Integer> map = new HashMap<>();
        if(n == 0) return "0";
        String sign = ((n > 0) ^ (d > 0)) ? "-":"";
        String ans = "";

        long numerator = n, denominator = d;
        numerator = Math.abs(numerator);
        denominator = Math.abs(denominator);

        // Calculate integer part
        long integer_part = numerator / denominator;
        ans = ans + Long.toString(integer_part);

        // Check if fractional part exists
        if(numerator % denominator != 0)
            ans = ans + ".";
        else
            return sign + ans;

        // Calculate fractional part
        for(long r = numerator % denominator; r != 0; r = r % denominator) {
            Integer lookup = map.get(r);
            if(lookup != null) {
                String part1 = ans.substring(0, lookup);
                String part2 = ans.substring(lookup, ans.length());
                ans = part1 + "(" + part2 + ")";
                break;
            }
            map.put(r, ans.length());
            r = r * 10;
            ans = ans + Long.toString(r / denominator);
        }

        return sign + ans;
    }
}