prevTail to keep track of the tail of the previous reversed group, currentHead to mark the start of the current group, and nextHead to mark the start of the next group.k nodes, reverse the group. Keep track of the new head and tail of the reversed group.prevTail to the new head of the reversed group.prevTail to the new tail of the reversed group.k nodes by setting currentHead to nextHead.k nodes, do not reverse it.