How to Find Union of Two Vectors in C++?

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v1 = {1, 7, 6, 6, 3};
    vector<int> v2 = {2, 5, 8, 4, 5};

    // Insert elements from both vectors to set
    set<int> res(v1.begin(), v1.end());
    res.insert(v2.begin(), v2.end());

    for (auto i : res)
        cout << i << " ";
    return 0;
}

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v1 = {1, 3, 6, 6, 7};
    vector<int> v2 = {2, 4, 5, 5, 8};

  	// Resultant vector to store union
    vector<int> res;

    // Finding union
    set_union(v1.begin(), v1.end(), v2.begin(),
              v2.end(), inserter(res, res.begin()));

    // Remove duplicates
    res.erase(unique(res.begin(), res.end()), res.end());

    for (auto i : res)
        cout << i << " ";
    return 0;
}

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v1 = {1, 3, 6, 6, 7};
    vector<int> v2 = {2, 4, 5, 5, 8};

    vector<int> res;
    unordered_set<int> seen;

    // Iterate through the first vector
    for (int i : v1) {
      
      	// Add i if not already seen and mark
      	// it as seen
        if (seen.find(i) == seen.end()) {
            res.push_back(i);  
            seen.insert(i);
        }
    }

    // Iterate through the second vector
    for (int i : v2) {
      
      	// Add i if not already seen and mark
    	// it as seen
        if (seen.find(i) == seen.end()) {
            res.push_back(i);
            seen.insert(i);
        }
    }

    for (auto i : res)
        cout << i << " ";
    return 0;
}