leetcode-504 十进制转七进制

//测试：100--->202  -7---->-10
 string convertToBase7(int num) {
    int res=0;
    string s;
    if(num<0){
        s="-";
    }
    int n=abs(num);
	 vector<int> v;
	 while(n!=0){
		 int remainder=n%7;
		 v.push_back(remainder);
		 n=n/7;
	 }
	 reverse(v.begin(), v.end());
	 for(int i=0;i<v.size();i++){
		 res=res*10+v[i];
	 }
    if(num<0){
        return s.append(to_string(static_cast<long long>(res)));
    }
	 else{
         return to_string(static_cast<long long>(res));
     }
   }