Given the head of a singly linked list and an integer k, reverse every consecutive group of k nodes. If the number of nodes remaining at the end isn't a full group of k, leave that final partial group exactly as it is. The optimal solution never reverses a group speculatively — it always peeks k nodes ahead with a throwaway check pointer first, confirming a full group exists before touching a single .nextLook Before You LeapReversing pointers is easy to do but hard to undo cleanly mid-way. Confirming the precondition (a full group of k nodes) BEFORE any relinking starts means the algorithm never has to detect a failure partway through and unwind it — by the time reversal begins, success is already guaranteed. pointer. Once confirmed, each group is reversed with the same three-pointer dance used to reverse an entire list, and a trailing groupPrev — anchored at the previous group's original first node, now its tail — links each freshly-reversed group to the next.