Sorting to obtain the Largest number

Given a list of non-negative integers, I have to arrange them such that they form the largest number and return the number as a string.
Input is[3, 30, 34, 5, 9]. I tried to traverse the array manually but I didn't understand where I was wrong. All the elements are swapped as I expected but 3 and 30 didn't get swapped. Could anyone please find the error in the code?

  #include <iostream>
   #include <vector>


  using namespace std;

   int main()
   {vector<int>A={ 3, 30, 34, 5, 9 };
  vector<int>b;b.assign(A.begin(),A.end());

   for(int i=0;i<b.size()-1;i++){

    for(int j=i+1;j<b.size();j++){

         string str1,str2,str3,str4;

         str1=to_string(A[i]);

         str2=to_string(A[j]);

         str3=str1.append(str2);


          str4=str2.append(str1);

           if(stoi(str4)>stoi(str3)){

             swap(b[i],b[j]);

         }

        }
     }
       string s="";
      for(int i=0;i<A.size();i++){
      s.append(to_string(b[i]));

        }
           cout<<s;

      return 0;
       }

   Expected 9534330
  Actual  9534303