class Solution{
    //Function to reverse any part of the array.
    void reverse(ArrayList<Integer> arr, int n,int left, int right)
    {
           //reversing the sub-array from left index to right index.
            while (left < right) { 
                //swapping values at index stored at left and right index.
                int temp = arr.get(left); 
                arr.set(left, arr.get(right)); 
                arr.set(right, temp);
                
                //updating values of left and right index.
                left+=1; 
                right-=1;  
            }
    }
    
    //Function to reverse every sub-array group of size k.
    void reverseInGroups(ArrayList<Integer> arr, int n, int k) {
        for (int i = 0; i < n; i += k) { 
            
            //If (ith index +k) is less than total number of elements it means
            //k elements exist from current index so we reverse k elements 
            //starting from current index.
            if(i+k < n){ 
                //reverse function called to reverse any part of the array.
                reverse(arr,n,i,i+k-1);
            }
            //Else k elements from current index doesn't exist. 
            //In that case we just reverse the remaining elements.
            else{
                //reverse function called to reverse any part of the array.
                reverse(arr,n,i,n-1);
            }
           
        }
    }
}