166 分数到小数

https://leetcode.cn/leetbook/read/top-interview-questions-medium/xwm8ne/

cost: 17m05s

class Solution {
public:
    string fractionToDecimal(int numerator, int denominator) {
        return this->fractionToDecimal_(numerator, denominator);
    }

private:
    string fractionToDecimal_(long numerator, long denominator) {
        string s = "";
        long r = numerator / denominator;
        if (r == 0 && ((numerator < 0 && denominator > 0) ||
                       (numerator > 0 && denominator < 0))) {
            s += "-";
        }
        s += to_string(r);
        if (numerator % denominator == 0) {
            return s;
        }
        s += ".";
        string s2 = "";
        numerator %= denominator;
        unordered_map<long, long> m;
        while (numerator != 0) {
            if (m.count(numerator)) {
                long i = m[numerator];
                s += s2.substr(0, i) + "(" + s2.substr(i) + ")";
                return s;
            }
            m[numerator] = s2.size();
            long tmp = numerator * 10 / denominator;
            s2 += to_string(tmp < 0 ? -tmp : tmp);
            numerator = numerator * 10 % denominator;
        }
        return s + s2;
    }
};