Consider the list structure ...A → (B → ... → C) → D..., where the nodes between B and C (inclusive) form a group, A is the last node of the previous group, and D is the first node of the next group. How can you utilize this structure?

Suppose you have B → ... → C reversed (because it was of even length) so that it is now C → ... → B. What references do you need to fix so that the transitions between the previous, current, and next groups are correct?

A.next should be set to C, and B.next should be set to D.

Once the current group is finished being modified, you need to find the new A, B, C, and D nodes for the next group. How can you use the old A, B, C, and D nodes to find the new ones?

The new A is either the old B or old C depending on if the group was of even or odd length. The new B is always the old D. The new C and D can be found based on the new B and the next group's length.

You can set the initial values of A, B, C, and D to A = null, B = head, C = head, D = head.next. Repeat the steps from the previous hints until D is null.