LC 592. Fraction Addition and Subtraction

class Solution {
public:
    long long gcd(long long a, long long b) {
        if(a < b) return gcd(b,a);
        if(b == 0) return a;
        return gcd(b, a%b);
    }
    string fractionAddition(string expression) {
        vector<int> numerator;
        vector<int> denominator;
        int j = 0;
        bool positive = false;
        if(expression[0] >= ‘0‘ && expression[0] <= ‘9‘) positive = true;
        else {
            positive = false;
            j = 1;
        }
        string tmp;
        for(size_t i=j+1; i<expression.size(); i++) {
            if(expression[i] == ‘+‘ || expression[i] == ‘-‘) {
                tmp = expression.substr(j, i-j);
                size_t k;
                for(k = 0; k < tmp.size(); k++) {
                    if(tmp[k] == ‘/‘) break;
                }
                numerator.push_back(stoi(tmp.substr(0,k)));
                if(!positive) numerator[numerator.size()-1] *= -1;
                positive = expression[i] == ‘+‘ ? true : false;
                denominator.push_back(stoi(tmp.substr(k+1)));
                j = i+1;
            }
        }
        tmp = expression.substr(j,expression.size()-j);
        size_t k;
        for(k=0; k<tmp.size(); k++)
            if(tmp[k] == ‘/‘) break;
        numerator.push_back(stoi(tmp.substr(0,k)));
        if(!positive) numerator[numerator.size()-1] *= -1;
        denominator.push_back(stoi(tmp.substr(k+1)));
// test




        long long ret_numer = numerator[0];
        long long ret_denomi = denominator[0];
        long long r = 0;
        for(size_t i= 1; i<denominator.size(); i++) {
            long long tmp_deno = ret_denomi;
            ret_denomi *= denominator[i];
            ret_numer = ret_numer * denominator[i] + tmp_deno * numerator[i];
        }
        if(ret_numer > 0) {
            r = gcd(ret_numer, ret_denomi);
            ret_numer /= r;
            ret_denomi /= r;
        } else if (ret_numer < 0) {
            r = gcd(-ret_numer, ret_denomi);
            ret_numer /= r;
            ret_denomi /= r;
        } else if(ret_numer == 0) {
            ret_denomi = 1;
        }
        string str_numer = ret_numer < 0 ? ("-" + to_string(-ret_numer)) : to_string(ret_numer);
        string str_deno  = to_string(ret_denomi);
        return str_numer + "/" + str_deno;
  }
};

class Solution {
public:
    string fractionAddition(string expression) {
        istringstream iss(expression);
        int num = 0, den = 0, NUM = 0, DEN = 1;
        char c;
        while (iss >> num >> c >> den)
        {
            NUM = NUM * den + num * DEN;
            DEN *= den;
            int g = abs(gcd(NUM, DEN));
            NUM /= g;
            DEN /= g;
        }
        
        return to_string(NUM) + "/" + to_string(DEN);
    }
    
    int gcd(int x, int y)
    {
        return y == 0 ? x : gcd(y, x % y);
    }
};