Multiply two strings

/*You are required to complete below function */
string multiplyStrings(string num1, string num2) {
    int nn=1,mm=1;  
    if(num1[0]=='-')
      { nn=-1; }
    if(num2[0]=='-')
      { mm=-1; }
    
    int mul= nn*mm;
    int n1 = num1.size();
    int n2 = num2.size();
    if (n1 == 0 || n2 == 0)     return "0";
    
    // will keep the result number in vector in reverse order
    vector<int> result(n1 + n2, 0);
    
    int i_n1 = 0; // index by num1
    int i_n2 = 0; // index by num2
    
    // go from right to left by num1
    for (int i = n1 - 1; i >= 0; i--){
        if(num1[i]=='-')
          continue;
        int carrier = 0;
        int n1 = num1[i] - '0';
        i_n2 = 0;
    
        // go from right to left by num2
        for (int j = n2 - 1; j >= 0; j--){
            if(num2[j]=='-')
               continue;
            int n2 = num2[j] - '0';     
    
            int sum = n1 * n2 + result[i_n1 + i_n2] + carrier;
            carrier = sum / 10;
            result[i_n1 + i_n2] = sum % 10;
    
            i_n2++;
        }
    
        // store carrier in next cell
        if (carrier > 0)
            result[i_n1 + i_n2] += carrier;
    
        i_n1++;
    }
    
    // ignore '0's from the right
    int i = result.size() - 1;
    while (i>=0 && result[i] == 0) i--;
    
    // if all were '0's - means either both or one of num1 or num2 were '0'
    if(i == -1) return "0";
    
    // generate the result string
    string s = "";
    string p;
    
    while (i >= 0) s += std::to_string(result[i--]);
    if(mul==-1)
    { p="-"; p.append(s); }
    else
       p.append(s);
    return p;
}