Solution.java

class Solution {
    public String fractionToDecimal(int numerator, int denominator) {
        if (numerator == 0) {
            return "0";
        }
        StringBuilder sb = new StringBuilder();
        boolean neg = (numerator > 0) ^ (denominator > 0);
        sb.append(neg ? "-" : "");
        long a = Math.abs((long) numerator), b = Math.abs((long) denominator);
        sb.append(a / b);
        a %= b;
        if (a == 0) {
            return sb.toString();
        }
        sb.append(".");
        Map<Long, Integer> d = new HashMap<>();
        while (a != 0) {
            d.put(a, sb.length());
            a *= 10;
            sb.append(a / b);
            a %= b;
            if (d.containsKey(a)) {
                sb.insert(d.get(a), "(");
                sb.append(")");
                break;
            }
        }
        return sb.toString();
    }
}