The reader is advised to read all the lessons (tutorials) in this full course, in the order presented.

Problem

Given an array, arr[] and an integer k=3, find the array after reversing every sub-array of consecutive k elements in place. If the last sub-array has fewer than k elements, reverse it as it is. Modify the array in place; do not return anything. The given array is:

    [0, 1, 2, 3, 4, 5, 6, 7]  

The algorithm should use O(n) time and O(n) space. The output should be:

    [2, 1, 0, 5, 4, 3, 7, 6]  

Solution

Edge Cases:

When k = 1, or k = 0, the array stays the same.
When k is greater than or equal to the array size, the whole array is reversed. 

Strategy

- Take the first edge case into consideration.
- Begin from index 0 and find the size of the current sub-array to be reversed. If the number of elements is less than k, reverse all of them.
- Each sub-array is reversed using two pointers (optionally) that start from the two corners of the sub-array.

In the function there is the principal for-loop, where increment of the index is done in k's.

Zero based indexing is used. The left included index of a group (sub-array) is obtained from,

    int left = i;

where i is the iterating index. The right included index of a group (sub-array) is obtained from,

    i+k-1    //zero based counting

where i is the iterating index (zero based).

The following program illustrates this (read through the code and comments):

<?php

    function reverseArrayInGroups(&$arr, $k) {    // note the use of &
        $n = count($arr);
    
        for ($i = 0; $i < $n; $i += $k) {
            $left = $i;
            $right;
    
            // to determine right value
            if($i+$k-1 < $n-1)
                $right = $i+$k-1;    //zero based indexing
            else
                $right = $n-1;    //zero based indexing

            // reverse the sub-array [left, right]
            while ($left < $right) {
                // swap
                $temp = $arr[$left];
                $arr[$left] = $arr[$right];
                $arr[$right] = $temp;

                $left++;
                $right--;
            }
        }
    }

  //main area
    $arr = [0, 1, 2, 3, 4, 5, 6, 7];
    $k = 3;
        
    reverseArrayInGroups($arr, $k);
        
    for ($i = 0; $i < count($arr); $i++) 
        echo($arr[$i] . " ");
    echo PHP_EOL;

?>

The output is:

    2 1 0 5 4 3 7 6 

Thanks for reading.