Merge Without Extra Space.cpp

/*
Merge Without Extra Space
=========================

Given two sorted arrays arr1[] of size N and arr2[] of size M. Each array is sorted in non-decreasing order. Merge the two arrays into one sorted array in non-decreasing order without using any extra space.

Example 1:
Input:
N = 4, M = 5
arr1[] = {1, 3, 5, 7}
arr2[] = {0, 2, 6, 8, 9}
Output: 0 1 2 3 5 6 7 8 9
Explanation: Since you can't use any 
extra space, modify the given arrays
to form 
arr1[] = {0, 1, 2, 3}
arr2[] = {5, 6, 7, 8, 9}

Example 2:
Input:
N = 2, M = 3
arr1[] = {10, 12}
arr2[] = {5, 18, 20}
Output: 5 10 12 18 20
Explanation: Since you can't use any
extra space, modify the given arrays
to form 
arr1[] = {5, 10}
arr2[] = {12, 18, 20}

Your Task:
You don't need to read input or print anything. Complete the function merge() which takes the two arrays arr1[], arr2[] and their sizes n and m, as input parameters. The function does not return anything. Use the given arrays to sort and merge arr1[] and arr2[] in-place. 
Note: The generated output will print all the elements of arr1[] followed by all the elements of arr[2].

Expected Time Complexity: O((n+m)*log(n+m))
Expected Auxiliary Space: O(1)

Constraints:
1 <= N, M <= 5*104
0 <= arr1i, arr2i <= 106
*/

int nextGap(int gap)
{
  if (gap <= 1)
    return 0;
  return (gap / 2) + (gap % 2);
}

void merge(int arr1[], int arr2[], int n, int m)
{
  int i, j, gap = n + m;
  for (gap = nextGap(gap); gap > 0; gap = nextGap(gap))
  {
    // comparing elements in the first array.
    for (i = 0; i + gap < n; i++)
      if (arr1[i] > arr1[i + gap])
        swap(arr1[i], arr1[i + gap]);

    // comparing elements in both arrays.
    for (j = gap > n ? gap - n : 0; i < n && j < m; i++, j++)
      if (arr1[i] > arr2[j])
        swap(arr1[i], arr2[j]);

    if (j < m)
      // comparing elements in the second array.
      for (j = 0; j + gap < m; j++)
        if (arr2[j] > arr2[j + gap])
          swap(arr2[j], arr2[j + gap]);
  }
}