Reverse Nodes in K Group

Verify a complete block first\n\nPut a dummy node before head and let groupPrev be the node preceding the next candidate group. From groupPrev, walk k links to find kth. If it does not exist, the remaining suffix has fewer than k nodes and must stay unchanged. Save groupNext = kth.next, then reverse nodes from groupPrev.next up to, but not including, groupNext. A convenient loop starts prev = groupNext and repeatedly points curr.next at prev; when it finishes, the old first node is the new tail. Connect groupPrev.next to kth, then move groupPrev to the old first node. The invariant is that the prefix before groupPrev is already finalized in k-sized reversed blocks. Every node is rewired once, giving O(n) time and O(1) extra space. Test k = 1, list length exactly k, a final incomplete group, and two adjacent groups.