Code:
#include <string>
#include <iostream>
using namespace std;
int main(){
    string s1="123"; //Set up string 1
    string s2="456"; //Set up string 2
    string s3;       //Setup string 3
 
    cout<<"First  string: "<<s1<<endl;
    cout<<"Second string: "<<s2<<endl;
    s3=s1;           //Copy string 1 into string 3
    for(int i=0;i<s2.size();i++){        //Loop through s2
        char c=s2[i]; //Grab current character from s2
        bool is_c_in_s3=true;
        for(int j=0;j<s3.size();j++){    //Loop though s3
            cout<<"Figure this portion out yourself."<<endl;
        }
        if(is_c_in_s3){
            s3+=c; //Add the element to String 3
        }
    }
    cout<<"Final string: "<<s3<<endl;
}

I need help from you. I try to write the code in C++ which return union of two strings , This is the code, if you know how to write the right code please help me...