Odd Even Linked List

Given the `head` of a singly linked list, group all the nodes with odd indices together followed by the nodes with even indices, and return _the reordered list_. The **first** node is considered **odd**, and the **second** node is **even**, and so on. Note that the relative order inside both the even and odd groups should remain as it was in the input. You must solve the problem in `O(1)` extra space complexity and `O(n)` time complexity.   **Example 1:** **Input:** head = [1,2,3,4,5] **Output:** [1,3,5,2,4] ``` **Example 2:** **Input:** head = [2,1,3,5,6,4,7] **Output:** [2,3,6,7,1,5,4] ```   **Constraints:** - The number of nodes in the linked list is in the range `[0, 104]`. - `-106 <= Node.val <= 106`