20220606数学：分数到小数

        题目描述：给定两个整数，分别表示分数的分子 numerator 和分母 denominator，以 字符串形式返回小数 。如果小数部分为循环小数，则将循环的部分括在括号内。如果存在多个答案，只需返回 任意一个 。对于所有给定的输入，保证 答案字符串的长度小于 10^4 。

        编码实现：

    public String fractionToDecimal(int numerator, int denominator) {
        if (denominator == 0) {
            return "";
        }
        if (numerator == 0) {
            return "0";
        }
        StringBuilder result = new StringBuilder();
        long num = numerator,den = denominator;

        if ((num >= 0) ^ (den >= 0)) {
            result.append("-");
        }
        num = Math.abs(num);
        den = Math.abs(den);

        result.append(num / den);
        num %= den;
        if (num == 0) {
            return result.toString();
        }

        result.append(".");
        int index = result.length() - 1;
        Map<Long, Integer> record = new HashMap<>();
        while (num != 0 && !record.containsKey(num)) {
            record.put(num, ++index);
            num *= 10;
            result.append(num / den);
            num %= den;
        }
        if (record.containsKey(num)) {
            result.insert(record.get(num), "(");
            result.append(")");
        }
        return result.toString();
    }