Merge Sort is a Divide and Conquer algorithm. It repeatedly divides the array into two halves and combines them in sorted manner. 

Given an array arr[], its starting position l and its ending position r. Merge Sort is achieved using the following algorithm. 

MergeSort(arr[], l,  r)
    If r > l
         1. Find the middle point to divide 
            the array into two halves:  
                 middle m = (l+r)/2
         2. Call mergeSort for first half:   
                 Call mergeSort(arr, l, m)
         3. Call mergeSort for second half:
                 Call mergeSort(arr, m+1, r)
         4. Merge the two halves sorted in 
            step 2 and 3:
                 Call merge(arr, l, m, r)

Implement the merge() function used in MergeSort().

Example 1:

Input:
N = 5
arr[] = {4 1 3 9 7}
Output: 1 3 4 7 9

Example 2:

Input:
N = 10
arr[] = {10 9 8 7 6 5 4 3 2 1}
Output: 1 2 3 4 5 6 7 8 9 10

Your Task:
You don't need to take the input or print anything. Your task is to complete the function merge() which takes arr[], l, m, r as its input parameters and modifies arr[] in-place such that it is sorted from position l to position r. 
Assume that the range [l,m] and [m+1,r] are already sorted.

Expected Auxiliary Space: O(n)
Expected Time Complexity: O(n) for the merge() function only.

Constraints:
1 <= N <= 10⁵
1 <= arr[i] <= 10³