Reverse array in groups

Source: Dev.to

Problem Statement

Reverse an array in groups of a given size k.
The array is divided into consecutive chunks (windows) of length k, and each chunk is reversed independently. The last chunk may contain fewer than k elements; it should also be reversed.

Approach

1. Iterate over the array in steps of k (indices 0, k, 2k, …).
2. For each starting index i, reverse the sub‑array arr[i : i+k].
3. Continue until the end of the array is reached.

The slice operation automatically handles the edge case where the remaining elements are fewer than k.

Implementation (Python)

def reverse_in_groups(arr, k):
    """
    Reverse the elements of `arr` in groups of size `k`.

    Parameters
    ----------
    arr : list
        The list to be processed.
    k : int
        The size of each group.

    Returns
    -------
    list
        The list with each group reversed.
    """
    n = len(arr)
    for i in range(0, n, k):
        # Reverse the sub-array from i to i+k
        arr[i:i + k] = reversed(arr[i:i + k])
    return arr

Complexity

• Time: O(n) – each element is visited at most twice (once during iteration and once during reversal).
• Space: O(1) additional space (the reversal is done in‑place).

Example

arr = [1, 2, 4, 5, 7]
k = 3

• Chunk 1: [1, 2, 4] → [4, 2, 1]
• Chunk 2 (edge case): [5, 7] → [7, 5]

Result: [4, 2, 1, 7, 5]