Reverse K Groups

Use a dummy node before the list and let groupPrev mark the node immediately before the next block. First walk k steps from groupPrev to find kth. If fewer than k nodes remain, stop and return dummy.next; the trailing partial group must remain unchanged. Save groupNext = kth.next. Reverse the nodes in the block by initializing prev = groupNext and curr = groupPrev.next, then repeatedly save curr.next, point curr.next to prev, and advance both pointers until curr == groupNext.

After reversal, the old first node is the tail. Save it before reversal as oldGroupHead = groupPrev.next; set groupPrev.next = kth, then set groupPrev = oldGroupHead to begin the next group. The invariant is that the prefix ending at groupPrev contains only fully reversed blocks and remains connected to the untouched suffix. Test k = 1, list length below k, exact multiples, a final short group, and empty input. Each node participates in at most one reversal, so time is O(n) and auxiliary space is O(1).