Fraction to Recurring Decimal

Q:

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)".

T:
Keep a remainder. Multiply it by 10 each time we append a new digit to the result, and append the quotient as the new digit, and assign remainder as the mod of the remainder to the denominator. We need a hashmap to store the positions of each remainder. When we see a remainder again, we know the digits will cycle.

A:

class Solution {
public:
    string fractionToDecimal(int numerator, int denominator) {
        int sign = 0;
        if((numerator < 0 && denominator > 0) || (numerator > 0 && denominator < 0)) {
            sign = 1;
        }
        int64_t n = numerator;
        int64_t d = denominator;
        n = abs(n);
        d = abs(d);
        string ret = "";
        if(sign) {
            ret += "-";
        }
        ret += to_string(n/d);
        if(n%d == 0) {
            return ret;
        }
        ret += ".";
        unordered_map< int, int > mark;
        int64_t r = n%d;
        while(r > 0) {
            if(mark[r] > 0) {
                ret.insert(mark[r], 1, '(');
                ret += ")";
                return ret;
            }
            mark[r] = ret.size();
            r *= 10;
            ret += to_string(r/d);
            r %= d;
        }
        return ret;
    }
};