What is the time and space complexity of reversing a singly linked list in groups of k?

• A. Time O(n^2); Space O(n)
• B. Time O(1); Space O(1)
• C. Time O(n); Space O(1) auxiliary
• D. Time O(n); Space O(n)

Answer: (C)

Understanding how reversing a singly linked list in groups of k works in terms of time and space.

You reverse each consecutive block of k nodes by rewiring the next pointers within that block and then connect the reversed blocks together. Each node is visited and its pointers adjusted a constant number of times, so the total amount of work scales linearly with the number of nodes in the list. That gives a time complexity of O(n).

Only a few pointers are needed to manage the reversal (such as pointers to the current node, the previous node within the group, and the heads and tails of groups). No extra data structures are created, so the auxiliary space used is constant, i.e., O(1). If the final group has fewer than k nodes, many implementations leave it in its current order; this does not affect the overall space usage, which remains constant.

Thus, the best answer reflects time complexity O(n) and space complexity O(1) auxiliary.