C Program for Iterative Merge Sort

Following is a typical recursive implementation of Merge Sort that uses last element as pivot. 

C

/* Recursive C program for merge sort */
#include <stdio.h>
#include <stdlib.h>

/* Function to merge the two haves arr[l..m] and arr[m+1..r] of array arr[] */
void merge(int arr[], int l, int m, int r);

/* l is for left index and r is right index of the sub-array
of arr to be sorted */
void mergeSort(int arr[], int l, int r)
{
 if (l < r) {
  int m = l + (r - l) / 2; // Same as (l+r)/2 but avoids overflow for large l & h
  mergeSort(arr, l, m);
  mergeSort(arr, m + 1, r);
  merge(arr, l, m, r);
 }
}

/* Function to merge the two haves arr[l..m] and arr[m+1..r] of array arr[] */
void merge(int arr[], int l, int m, int r)
{
 int i, j, k;
 int n1 = m - l + 1;
 int n2 = r - m;

 /* create temp arrays */
 int L[n1], R[n2];

 /* Copy data to temp arrays L[] and R[] */
 for (i = 0; i < n1; i++)
  L[i] = arr[l + i];
 for (j = 0; j < n2; j++)
  R[j] = arr[m + 1 + j];

 /* Merge the temp arrays back into arr[l..r]*/
 i = 0;
 j = 0;
 k = l;
 while (i < n1 && j < n2) {
  if (L[i] <= R[j]) {
   arr[k] = L[i];
   i++;
  }
  else {
   arr[k] = R[j];
   j++;
  }
  k++;
 }

 /* Copy the remaining elements of L[], if there are any */
 while (i < n1) {
  arr[k] = L[i];
  i++;
  k++;
 }

 /* Copy the remaining elements of R[], if there are any */
 while (j < n2) {
  arr[k] = R[j];
  j++;
  k++;
 }
}

/* Function to print an array */
void printArray(int A[], int size)
{
 int i;
 for (i = 0; i < size; i++)
  printf("%d ", A[i]);
 printf("\n");
}

/* Driver program to test above functions */
int main()
{
 int arr[] = { 12, 11, 13, 5, 6, 7 };
 int arr_size = sizeof(arr) / sizeof(arr[0]);

 printf("Given array is \n");
 printArray(arr, arr_size);

 mergeSort(arr, 0, arr_size - 1);

 printf("\nSorted array is \n");
 printArray(arr, arr_size);
 return 0;
}

Output:

Given array is 
12 11 13 5 6 7 

Sorted array is 
5 6 7 11 12 13

Time Complexity: O(n*log(n))
Auxiliary Space: O(n)

Please refer complete article on Iterative Merge Sort for more details!