Reverse Nodes in k-Group

Use a dummy node before the head so the first group has an ordinary predecessor. From groupPrev, find the kth node; if fewer than k nodes remain, stop and leave the suffix unchanged. Save groupNext = kth.next, then reverse nodes from the group head up to but excluding groupNext. During reversal, initialise prev to groupNext so the old head naturally points to the following group. After reversal, the old group head is the new tail: reconnect groupPrev.next to kth, then move groupPrev to that old head. The invariant after each iteration is that the prefix through groupPrev is correctly k-reversed and the remaining suffix is untouched. Test k=1, k greater than length, exact multiples, a trailing short group, and an empty list. Each node is visited a constant number of times, so time is O(n) and extra space O(1).