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Merge Sort with O(1) extra space merge and O(n lg n) time
  • Difficulty Level : Expert
  • Last Updated : 18 Dec, 2020

We have discussed Merge sort. How to modify the algorithm so that merge works in O(1) extra space and algorithm still works in O(n Log n) time. We may assume that the input values are integers only.

Examples: 

Input : 5 4 3 2 1
Output : 1 2 3 4 5

Input : 999 612 589 856 56 945 243
Output : 56 243 589 612 856 945 999

For integer types, merge sort can be made inplace using some mathematics trick of modulus and division. That means storing two elements value at one index and can be extracted using modulus and division. 

First we have to find a value greater than all the elements of the array. Now we can store the original value as modulus and the second value as division. Suppose we want to store arr[i] and arr[j] both at index i(means in arr[i]). First we have to find a ‘maxval’ greater than both arr[i] and arr[j]. Now we can store as arr[i] = arr[i] + arr[j]*maxval. Now arr[i]%maxval will give the original value of arr[i] and arr[i]/maxval will give the value of arr[j]. So below is the implementation on merge sort.

C++




// C++ program to sort an array using merge sort such
// that merge operation takes O(1) extra space.
#include <bits/stdc++.h>
using namespace std;
void merge(int arr[], int beg, int mid, int end, int maxele)
{
    int i = beg;
    int j = mid + 1;
    int k = beg;
    while (i <= mid && j <= end) {
        if (arr[i] % maxele <= arr[j] % maxele) {
            arr[k] = arr[k] + (arr[i] % maxele) * maxele;
            k++;
            i++;
        }
        else {
            arr[k] = arr[k] + (arr[j] % maxele) * maxele;
            k++;
            j++;
        }
    }
    while (i <= mid) {
        arr[k] = arr[k] + (arr[i] % maxele) * maxele;
        k++;
        i++;
    }
    while (j <= end) {
        arr[k] = arr[k] + (arr[j] % maxele) * maxele;
        k++;
        j++;
    }
 
    // Obtaining actual values
    for (int i = beg; i <= end; i++)
        arr[i] = arr[i] / maxele;
}
 
// Recursive merge sort with extra parameter, naxele
void mergeSortRec(int arr[], int beg, int end, int maxele)
{
    if (beg < end) {
        int mid = (beg + end) / 2;
        mergeSortRec(arr, beg, mid, maxele);
        mergeSortRec(arr, mid + 1, end, maxele);
        merge(arr, beg, mid, end, maxele);
    }
}
 
// This functions finds max element and calls recursive
// merge sort.
void mergeSort(int arr[], int n)
{
   int maxele = *max_element(arr, arr+n) + 1;
   mergeSortRec(arr, 0, n-1, maxele);
}
 
int main()
{
    int arr[] = { 999, 612, 589, 856, 56, 945, 243 };
    int n = sizeof(arr) / sizeof(arr[0]);
     
    mergeSort(arr, n);
 
    cout << "Sorted array \n";
    for (int i = 0; i < n; i++)
        cout << arr[i] << " ";
    return 0;
}

Java




// Java program to sort an array
// using merge sort such that
// merge operation takes O(1)
// extra space.
import java.util.Arrays;
 
class GFG
{
 
    static void merge(int[] arr, int beg,
                        int mid, int end,
                        int maxele)
    {
        int i = beg;
        int j = mid + 1;
        int k = beg;
        while (i <= mid && j <= end)
        {
            if (arr[i] % maxele <=
                arr[j] % maxele)
            {
                arr[k] = arr[k] + (arr[i]
                        % maxele) * maxele;
                k++;
                i++;
            }
            else
            {
                arr[k] = arr[k] +
                        (arr[j] % maxele)
                        * maxele;
                k++;
                j++;
            }
        }
        while (i <= mid)
        {
            arr[k] = arr[k] + (arr[i]
                    % maxele) * maxele;
            k++;
            i++;
        }
        while (j <= end)
        {
            arr[k] = arr[k] + (arr[j]
                    % maxele) * maxele;
            k++;
            j++;
        }
 
        // Obtaining actual values
        for (i = beg; i <= end; i++)
        {
            arr[i] = arr[i] / maxele;
        }
    }
 
    // Recursive merge sort
    // with extra parameter, naxele
    static void mergeSortRec(int[] arr, int beg,
            int end, int maxele)
    {
        if (beg < end)
        {
            int mid = (beg + end) / 2;
            mergeSortRec(arr, beg,
                        mid, maxele);
            mergeSortRec(arr, mid + 1,
                        end, maxele);
            merge(arr, beg, mid,
                    end, maxele);
        }
    }
 
    // This functions finds
    // max element and calls
    // recursive merge sort.
    static void mergeSort(int[] arr, int n)
    {
        int maxele = Arrays.stream(arr).max().getAsInt() + 1;
        mergeSortRec(arr, 0, n - 1, maxele);
    }
 
    // Driver code
    public static void main(String[] args)
    {
 
        int[] arr = {999, 612, 589,
                    856, 56, 945, 243};
        int n = arr.length;
 
        mergeSort(arr, n);
 
        System.out.println("Sorted array ");
        for (int i = 0; i < n; i++)
        {
            System.out.print(arr[i] + " ");
        }
    }
}
 
// This code is contributed by 29AjayKumar

Python3




# Python3 program to sort an array using
# merge sort such that merge operation
# takes O(1) extra space.
def merge(arr, beg, mid, end, maxele):
     
    i = beg
    j = mid + 1
    k = beg
     
    while (i <= mid and j <= end):
        if (arr[i] % maxele <= arr[j] % maxele):
            arr[k] = arr[k] + (arr[i] %
                    maxele) * maxele
            k += 1
            i += 1
        else:
            arr[k] = arr[k] + (arr[j] %
                    maxele) * maxele
            k += 1
            j += 1
             
    while (i <= mid):
        arr[k] = arr[k] + (arr[i] %
                maxele) * maxele
        k += 1
        i += 1
    while (j <= end):
        arr[k] = arr[k] + (arr[j] %
                maxele) * maxele
        k += 1
        j += 1
 
    # Obtaining actual values
    for i in range(beg, end + 1):
        arr[i] = arr[i] // maxele
 
# Recursive merge sort with extra
# parameter, naxele
def mergeSortRec(arr, beg, end, maxele):
     
    if (beg < end):
        mid = (beg + end) // 2
        mergeSortRec(arr, beg, mid, maxele)
        mergeSortRec(arr, mid + 1, end, maxele)
        merge(arr, beg, mid, end, maxele)
 
# This functions finds max element and
# calls recursive merge sort.
def mergeSort(arr, n):
     
   maxele = max(arr) + 1
   mergeSortRec(arr, 0, n - 1, maxele)
 
# Driver Code
if __name__ == '__main__':
 
    arr = [ 999, 612, 589, 856, 56, 945, 243 ]
    n =  len(arr)
    mergeSort(arr, n)
 
    print("Sorted array")
     
    for i in range(n):
        print(arr[i], end = " ")
 
# This code is contributed by mohit kumar 29

C#




// C# program to sort an array
// using merge sort such that
// merge operation takes O(1)
// extra space.
using System;
using System.Linq;
 
class GFG
{
static void merge(int []arr, int beg,
                  int mid, int end,
                  int maxele)
{
    int i = beg;
    int j = mid + 1;
    int k = beg;
    while (i <= mid && j <= end)
    {
        if (arr[i] %
            maxele <= arr[j] % maxele)
        {
            arr[k] = arr[k] + (arr[i] %
                     maxele) * maxele;
            k++;
            i++;
        }
        else
        {
            arr[k] = arr[k] +
                    (arr[j] % maxele) *
                              maxele;
            k++;
            j++;
        }
    }
    while (i <= mid)
    {
        arr[k] = arr[k] + (arr[i] %
                 maxele) * maxele;
        k++;
        i++;
    }
    while (j <= end)
    {
        arr[k] = arr[k] + (arr[j] %
                 maxele) * maxele;
        k++;
        j++;
    }
 
    // Obtaining actual values
    for ( i = beg; i <= end; i++)
        arr[i] = arr[i] / maxele;
}
 
// Recursive merge sort
// with extra parameter, naxele
static void mergeSortRec(int []arr, int beg,
                         int end, int maxele)
{
    if (beg < end)
    {
        int mid = (beg + end) / 2;
        mergeSortRec(arr, beg,
                     mid, maxele);
        mergeSortRec(arr, mid + 1,
                     end, maxele);
        merge(arr, beg, mid,
              end, maxele);
    }
}
 
// This functions finds
// max element and calls
// recursive merge sort.
static void mergeSort(int []arr, int n)
{
    int maxele = arr.Max() + 1;
    mergeSortRec(arr, 0, n - 1, maxele);
}
 
//Driver code
public static void Main ()
{
    int []arr = {999, 612, 589,
                 856, 56, 945, 243};
    int n = arr.Length;
 
    mergeSort(arr, n);
 
    Console.WriteLine("Sorted array ");
    for (int i = 0; i < n; i++)
        Console.Write( arr[i] + " ");
}
}
 
// This code is contributed
// by inder_verma.
Output: 
Sorted array 
56 243 589 612 856 945 999

 

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