Fraction to Recurring Decimal -- LeetCode

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.

For example,

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

思路：模拟除法。用一个map记录在哪个位置余数开始出现重复，从而可以插入‘（’。

class Solution {
public:
    string fractionToDecimal(int numerator, int denominator) {
        if (numerator == 0) return "0";
        string res;
        //calculate the sign
        if (numerator < 0 ^ denominator < 0) res += '-';
        int64_t n = abs((int64_t)numerator);
        int64_t d = abs((int64_t)denominator);
        
        //get the value before '.'
        res += std::to_string(n / d);
        if (n % d == 0) return res;
        
        res += '.';
        //use map to keep index, so that we know where to insert the '('
        unordered_map<int64_t, int> remains;
        for (int64_t r = n % d; r; r %= d) {
            if (remains.count(r) > 0) {
                res.insert(remains[r], "(");
                res += ')';
                break;
            }
            remains.insert(make_pair(r, res.size()));
            r *= 10;
            res += std::to_string(r / d);
        }
        return res;
    }
};