There is a question in my lab i couldnt figure it out .. I need your help to give me hints not to solve it.

Code:

/* Mergesort: Use merge() to sort an array of size n */
#include <stdio.h>
#include <stdlib.h>
void mergesort(int key[], int n)
{
int j, k, m, *w;
for (m = 1; m < n; m *= 2)
;
if (m != n) {
printf("ERROR: Size of the array is not a power of 2 - bye!\n");
exit(1);
}
w = calloc(n, sizeof(int)); /* allocate workspace */
for (k = 1; k < n; k *= 2) {
for (j = 0; j < n - k; j += 2 * k)
merge(key + j, key + j + k, w + j, k, k); /* merge into w */
for (j = 0; j < n; ++j)
key[j] = w[j]; /* write w back into key */
}
free(w); /* free the workspace */
}
/* Merge a[] of size m and b[] of size n into c[].*/
void merge(int a[], int b[], int c[], int m, int n)
{
int i = 0, j = 0, k = 0;
while (i < m && j < n)
if (a[i] < b[j])
c[k++] = a[i++];
else
c[k++] = b[j++];
while (i < m) /* pick up any remainder */
c[k++] = a[i++];
while (j < n)
c[k++] = b[j++];
}

Question :

Modify mergesort() so that it can be used with an array of any size, not just with a size that is a power of two. Recall that any positive integer can be expressed as a sum of powers of two. For example,

27 = 16 + 8 + 2 + 1

Consider the array as a collection of subarrays of sizes that are powers of two. Sort the subarrays and then use merge() to produce the final sorted array.

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I tried so many algorithms and all failed!!

What i dont know is how to create subarrays ?? what is the idea ??