Fraction

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.

Given numerator = 1, denominator = 2, return "0.5"
Given numerator = 2, denominator = 1, return "2"
Given numerator = 2, denominator = 3, return "0.(6)"

Method:

When we see the same remainder twice, we know there is a repeating pattern in the decimal.

Solution:

Time: O(n)
Space: O(n)

public class Solution {
    public String fractionToDecimal(int A, int B) {
        boolean negative = (A < 0) ^ (B < 0);
        StringBuilder sb = new StringBuilder();
        long a = Math.abs((long) A);
        long b = Math.abs((long) B);
        long q = a / b;
        sb.append(q + "");
        if (b * q == a) {
            if (q == 0 || !negative) {
                return sb.toString();
            } else {
                sb.insert(0, "-");
                return sb.toString();
            }
        }
        sb.append(".");
        StringBuilder decimal = new StringBuilder();
        Map<Long, Integer> map = new HashMap<>();
        long repeating = -1;
        long r = a - b * q;
        while (r > 0) {
            if (map.containsKey(r)) {
                repeating = r;
                break;
            } else {
                map.put(r, decimal.length());
                r = r * 10;
                q = r / b;
                decimal.append(q + "");
                r = r - b * q;
            }
        }
        if (negative) sb.insert(0, "-");
        if (repeating != -1) decimal.insert(map.get(repeating), "(");
        sb.append(decimal.toString());
        if (repeating != -1) sb.append(")");
        return sb.toString();
    }
}