Which description correctly outlines the k-group reversal for a singly linked list?

• A. Repeatedly reverse blocks of size k by isolating each block, reversing pointers within the block, and reconnecting blocks; O(n) time, O(1) extra space beyond pointers
• B. Recursively reverse the entire list
• C. Use a stack to reverse every node
• D. Leave list unchanged

Answer: (A)

Reversing nodes in fixed-size blocks as you traverse the list and reconnecting those blocks is the essential idea. You walk through the list, take the next k nodes as a block, reverse the next pointers inside that block, and then attach the reversed block to the portion you've already processed. You repeat this for each block until you run out of nodes. If the final block has fewer than k nodes, you leave it in its original order. This works in linear time because every node is touched a constant number of times, and it uses only a few pointers, giving O(1) extra space beyond the existing nodes. Approaches that reverse the entire list or rely on extra data structures would either spend more space or reverse more than needed, while simply leaving the list unchanged wouldn’t achieve the requested k-group reversal.