C++ Program to Merge Two Sorted Arrays

Last Updated : 23 Aug, 2024

Given two sorted arrays, the task is to merge them in a sorted manner.
Examples:

Input: arr1[] = { 1, 3, 4, 5}, arr2[] = {2, 4, 6, 8}
Output: arr3[] = {1, 2, 3, 4, 4, 5, 6, 8}

Input: arr1[] = { 5, 8, 9}, arr2[] = {4, 7, 8}
Output: arr3[] = {4, 5, 7, 8, 8, 9}

Naive Approach:

It is the brute force method to do the same. Take all the elements of arr1 and arr2 in arr3. Then simply sort the arr3.

The implementation of above approach is:

C++

// C++ program to merge two sorted arrays/
#include<bits/stdc++.h>
using namespace std;

void mergeArrays(int arr1[], int arr2[], int n1,
                            int n2, int arr3[])
{
    int i = 0, j = 0, k = 0;
      // traverse the arr1 and insert its element in arr3
      while(i < n1){
      arr3[k++] = arr1[i++];
    }
      
      // now traverse arr2 and insert in arr3
      while(j < n2){
      arr3[k++] = arr2[j++];
    }
      
      // sort the whole array arr3
      sort(arr3, arr3+n1+n2);
}

// Driver code
int main()
{
    int arr1[] = {1, 3, 5, 7};
    int n1 = sizeof(arr1) / sizeof(arr1[0]);

    int arr2[] = {2, 4, 6, 8};
    int n2 = sizeof(arr2) / sizeof(arr2[0]);

    int arr3[n1+n2];
    mergeArrays(arr1, arr2, n1, n2, arr3);

    cout << "Array after merging" <<endl;
    for (int i=0; i < n1+n2; i++)
        cout << arr3[i] << " ";

    return 0;
}

Output

Array after merging
1 2 3 4 5 6 7 8

Time Complexity : O((m+n) log(m+n)) , the whole size of arr3 is m+n
Auxiliary Space: O(1), No extra space is used

Method 2 (O(n1 * n2) Time and O(n1+n2) Extra Space)

Create an array arr3[] of size n1 + n2.
Copy all n1 elements of arr1[] to arr3[]
Traverse arr2[] and one by one insert elements (like insertion sort) of arr3[] to arr1[]. This step take O(n1 * n2) time.

We have discussed implementation of above method in Merge two sorted arrays with O(1) extra space
Method 3 (O(n1 + n2) Time and O(n1 + n2) Extra Space)
The idea is to use Merge function of Merge sort.

Create an array arr3[] of size n1 + n2.
Simultaneously traverse arr1[] and arr2[].
- Pick smaller of current elements in arr1[] and arr2[], copy this smaller element to next position in arr3[] and move ahead in arr3[] and the array whose element is picked.
If there are remaining elements in arr1[] or arr2[], copy them also in arr3[].

Below image is a dry run of the above approach:

Below is the implementation of the above approach:

C++

// C++ program to merge two sorted arrays/
#include<iostream>
using namespace std;

// Merge arr1[0..n1-1] and arr2[0..n2-1] into
// arr3[0..n1+n2-1]
void mergeArrays(int arr1[], int arr2[], int n1,
                             int n2, int arr3[])
{
    int i = 0, j = 0, k = 0;

    // Traverse both array
    while (i<n1 && j <n2)
    {
        // Check if current element of first
        // array is smaller than current element
        // of second array. If yes, store first
        // array element and increment first array
        // index. Otherwise do same with second array
        if (arr1[i] < arr2[j])
            arr3[k++] = arr1[i++];
        else
            arr3[k++] = arr2[j++];
    }

    // Store remaining elements of first array
    while (i < n1)
        arr3[k++] = arr1[i++];

    // Store remaining elements of second array
    while (j < n2)
        arr3[k++] = arr2[j++];
}

// Driver code
int main()
{
    int arr1[] = {1, 3, 5, 7};
    int n1 = sizeof(arr1) / sizeof(arr1[0]);

    int arr2[] = {2, 4, 6, 8};
    int n2 = sizeof(arr2) / sizeof(arr2[0]);

    int arr3[n1+n2];
    mergeArrays(arr1, arr2, n1, n2, arr3);

    cout << "Array after merging" <<endl;
    for (int i=0; i < n1+n2; i++)
        cout << arr3[i] << " ";

    return 0;
}

Output

Array after merging
1 2 3 4 5 6 7 8

Output:

Array after merging
1 2 3 4 5 6 7 8

Time Complexity : O(n1 + n2)
Auxiliary Space : O(n1 + n2)

Method 4: Using Maps (O(nlog(n) + mlog(m)) Time and O(N) Extra Space)

Insert elements of both arrays in a map as keys.
Print the keys of the map.

Below is the implementation of above approach.

CPP

// C++ program to merge two sorted arrays 
//using maps 
#include<bits/stdc++.h>
using namespace std;

// Function to merge arrays
void mergeArrays(int a[], int b[], int n, int m) 
{
    // Declaring a map.
    // using map as a inbuilt tool
    // to store elements in sorted order.
    map<int, int> mp;
    
    // Inserting values to a map.
    for(int i = 0; i < n; i++)mp[a[i]]++;
    
    
    for(int i = 0;i < m;i++)mp[b[i]]++;

    
    // Printing keys of the map.
    for(auto j: mp)
    {
         for(int i=0; i<j.second;i++)cout<<j.first<<" ";
    }
}

// Driver Code
int main()
{ 
    int a[] = {1, 3, 5, 7}, b[] = {2, 4, 6, 8};
    
    int size = sizeof(a)/sizeof(int);
    int size1 = sizeof(b)/sizeof(int);
    
    // Function call
    mergeArrays(a, b, size, size1);
    
    return 0;
}

//This code is contributed by yashbeersingh42