leetcode[166]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)".

class Solution {
public:
string  to_str(long long  n)
{
    string str="";
    if(n==0)
    {
        str+='0';
        return str;
    }
    long long tmp=n;
    long long power=1;
    int icount=0;
    while (tmp)
    {
        tmp/=10;
        power*=10;
        icount++;
    }
    power/=10;
    tmp=n;
    for (int i=0;i<icount;i++)
    {
        long long d=tmp/power;
        str+=char('0'+d);
        tmp-=d*power;
        power/=10;
    }
    return str;
}
string fractionToDecimal(int numerator, int denominator) {
    if(denominator==0)return "";
    if(numerator==0)return "0";
    string res="";
    long long num=numerator,  den=denominator;
    if((num>0&&den<0)||(num<0&&den>0))
        res+="-";
    num=num>=0?num:-num,den=den>=0?den:-den;
    long long part=num/den;
    res+=to_str(part);
    long long f=num%den;
    if(f==0)return res;
    res+='.';
    map<int,int> fmap;
    while(f)
    {
        if (fmap.count(f))
        {
            int beg=fmap[f];
            string part1=res.substr(0,beg);
            string part2=res.substr(beg,res.length());
            res=part1+"("+part2+")";
            return res;
        } 
        else
        {
            fmap[f]=res.length();            
            f*=10;
            res+=to_str(f/den);
            f%=den;        
        }
    }
    return res;
}
};