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

Last Updated : 29 Nov, 2024

The union of two vectors refers to the collection of all the distinct elements from the two vectors. In this article, we will learn how we can find the union of two sorted vectors in C++.

The most efficient method to find the union of two sorted vector is by using set_union() method. Let’s take a look at an example:

C++

#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;
}

Output

1 2 3 4 5 6 7 8

Explanation: The set_union() function works on sorted range and the vectors v1 and v2 are already sorted. The resultant vector res is then cleaned to remove duplicates.

There are also some other methods in C++ to find the union of two vector. Some of them are as follows:

Using Set

Insert all elements from both vectors into a set which by default stores only the unique elements in some defined order.

C++

#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};

    // Store v1 and v2 in set res
    set<int> res(v1.begin(), v1.end());
    res.insert(v2.begin(), v2.end());


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

Output

1 2 3 4 5 6 7 8

The problem with this method is that it may alter the order of elements of the result.

Using Two Pointer

The idea is to traverse the two vectors starting from the beginning using two pointers while adding unique elements to the resultant union vector. If the vectors are of unequal size, then add the remaining elements.

C++

#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;
    int i = 0, j = 0;

    while (i < v1.size() && j < v2.size()) {
        if (v1[i] < v2[j]) {
            
            // If element in v1 is smaller, add it to res
            if (res.empty() || res.back() != v1[i])
                res.push_back(v1[i]);
            i++;
        } else if (v1[i] > v2[j]) {
            
            // If element in v2 is smaller, add it to res
            if (res.empty() || res.back() != v2[j])
                res.push_back(v2[j]);
            j++;
        } else {
            
            // If elements are equal, add either one and
            // move both pointers
            if (res.empty() || res.back() != v1[i])
                res.push_back(v1[i]);
            i++;
            j++;
        }
    }

    // Add remaining elements of v1
    while (i < v1.size()) {
        if (res.empty() || res.back() != v1[i])
            res.push_back(v1[i]);
        i++;
    }

    // Add remaining elements of v2
    while (j < v2.size()) {
        if (res.empty() || res.back() != v2[j])
            res.push_back(v2[j]);
        j++;
    }

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

Output

1 2 3 4 5 6 7 8

How to Find the Median of Sorted Vector in C++?

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

Using Set

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