Fraction to Recurring Decimal

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)”.
Credits:
Special thanks to @Shangrila for adding this problem and creating all test cases.

思路：以2/7为例：

class Solution {
public:
    string to_string(long x){
        std::string number;
        std::stringstream strstream;
        strstream << x;
        strstream >> number;
        return number;
    }
    string fractionToDecimal(int num, int den) {//虽然声明为int,但测试的时候，会用大数
        if(num==0){
            return "0";
        }
        string sign="";//结果的符号
        if(((num>>31)^(den>>31))!=0){//异号
             sign="-";
        }
        long long numerator=abs((long long)num);//先转成long long,再用abs
        long long denominator=abs((long long)den);
        if(numerator%denominator==0){//能够整除
            return sign+to_string((numerator/denominator));
        }
        long n=numerator/denominator;//整数部分
        numerator=numerator%denominator;//分子
        map<int,int> m;
        long i=0;
        string s="";
        bool flag=false;
        while(m.find(numerator)==m.end()){
            m[numerator]=i;
            numerator*=10;
            i++;
            if(numerator==0){//能够除尽
                flag=true;
                break;
            }
            if(numerator<denominator){//不够除,补0
                s+="0";
            }else{
                int l=numerator/denominator;
                s+=to_string(l);
                numerator%=denominator;
            }

        }
        if(flag){
            return sign+to_string(n)+"."+s;
        }
        int index=m.find(numerator)->second;
        return sign+to_string(n)+("."+s.substr(0,index))+"("+s.substr(index,i-index)+")";
    }
};