Reverse Array in Groups

Reverse Array in Groups

Problem Statement

Given an array arr[] of positive integers of size N. Reverse every sub-array group of size K.
Note: If at any instance, there are no more subarrays of size greater than or equal to K, then reverse the last subarray (irrespective of its size).

Example 1

Input :
N = 5, K = 3
arr[] = {1,2,3,4,5}

Output : 3 2 1 5 4

Explanation :
First group consists of elements 1, 2, 3. Second group consists of 4,5.

Example 2

Input :
N = 4, K = 3
arr[] = {5,6,8,9}

Output : 8 6 5 9

Task

You don't need to read input or print anything. The task is to complete the function reverseInGroups() which takes the array, N and K as input parameters and modifies the array in-place.

Expected Time Complexity : O(N)
Expected Auxiliary Space : O(N)

Constraints :
1 <= N <= 10⁷
1 <= A[i] <= 10¹⁸

Solutions

Java Solution

class Solution {
    //Function to reverse every sub-array group of size k.
    void reverseInGroups(ArrayList<Integer> arr, int n, int k) {
        // code here
        for (int i=0; i<n; i=i+k) {
			int l = i;

			if (n - l >= k) {
				for (int j = i + k-1; j > l; j--, l++) {
					int temp = arr.get(l);
					arr.set(l, arr.get(j));
					arr.set(j, temp);
				}
			} else {
				for (int j = n-1; j > l; j--, l++) {
					int temp = arr.get(l);
					arr.set(l, arr.get(j));
					arr.set(j, temp);
				}
				break;
			}
        }
    }
}