C Program for Iterative Merge Sort

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

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/* 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;
}

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Output:

Given array is 
12 11 13 5 6 7 

Sorted array is 
5 6 7 11 12 13

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

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