Odd Even Linked List

Statement

Given the head of a singly linked list, rearrange the nodes so that all nodes at odd indexes appear first, followed by all nodes at even indexes.

• The first node in the list is considered at odd index 1, the second at even index 2, and so on.

• Within the odd group and the even group, the relative order of the nodes must remain the same as in the original list.

Return the head of the reordered linked list.

Note: You must solve the problem in $O(1)$ ...