Accepted 0ms c++ solution with std::unordered_map to record all the remainders, easy understand.

• After the decimal point, we need to record all the remainders and the corresponding position to insert `(`. When we get a new remainder, we should check whether it is already in the std::unordered_map. If true, it means the fractional part is repeating, so we can break the `for` loop, then append `)` and insert `(` to the corresponding position.

``````class Solution {
public:
std::string fractionToDecimal(long long numerator, long long denominator) {
if (!numerator)
return "0";
std::string str = numerator < 0 ^ denominator < 0 ? "-" : "";
numerator = std::abs(numerator);
denominator = std::abs(denominator);
str += std::to_string(numerator / denominator);
long long remainder = numerator % denominator;
if (!remainder)
return str;
str += '.';
std::unordered_map<int, int> flag;
int pos = str.size();
while (remainder && flag.find(remainder) == flag.end()) {
flag[remainder] = pos++;
str += '0' + remainder * 10 / denominator;
remainder = remainder * 10 % denominator;
}
if (remainder) {
str += ')';
str.insert(str.begin() + flag[remainder], '(');
}
return str;
}
};``````

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