166. Fraction to Recurring Decimal

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Problem

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)"

Code

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

        StringBuilder res = new StringBuilder();
        String sign = (numerator > 0) ^ (denominator > 0) ? "-" : "";
        res.append(sign);

        long num = Math.abs((long)numerator);
        long den = Math.abs((long)denominator);

        res.append(num / den);
        num %= den;

        if (num == 0) {
            return res.toString();
        }

        res.append(".");

        HashMap<Long, Integer> map = new HashMap<Long, Integer>();
        map.put(num, res.length());

        while (num != 0) {
            // 除法过程
            num *= 10;
            res.append(num / den);
            num %= den;
            if (map.containsKey(num)) {
                int index = map.get(num);
                res.insert(index, "(");
                res.append(")");
                break;
            } else {
                map.put(num, res.length());
            }
        }
        return res.toString();
    }
}