Reverse Nodes in Even Length Groups

Measure the actual group before deciding\n\nGroups have intended lengths 1, 2, 3, and so on, but the final group can be shorter. Keep prevTail before the current group and walk at most groupSize nodes to obtain count and the first node after the group. If count is odd, leave that section alone and set prevTail to its last node. If count is even, reverse exactly count nodes, connect prevTail.next to the new head, and connect the old group head, now its tail, to the saved next node. Then continue with groupSize + 1. The key invariant is that everything before prevTail is final and connected, while the remaining suffix is untouched. Each node is visited a constant number of times, so time is O(n) and auxiliary space is O(1). Test one node, a final short even group, a final short odd group, and a group whose reversal changes the list head.