public class Solution
extends Object
2074 - Reverse Nodes in Even Length Groups\.
Medium
You are given the `head` of a linked list.
The nodes in the linked list are **sequentially** assigned to **non-empty** groups whose lengths form the sequence of the natural numbers (`1, 2, 3, 4, ...`). The **length** of a group is the number of nodes assigned to it. In other words,
* The 1st node is assigned to the first group.
* The 2nd and the 3rd nodes are assigned to the second group.
* The 4th, 5th, and 6th nodes are assigned to the third group, and so on.
Note that the length of the last group may be less than or equal to `1 + the length of the second to last group`.
**Reverse** the nodes in each group with an **even** length, and return _the_ `head` _of the modified linked list_.
**Example 1:**
**Input:** head = [5,2,6,3,9,1,7,3,8,4]
**Output:** [5,6,2,3,9,1,4,8,3,7]
**Explanation:**
- The length of the first group is 1, which is odd, hence no reversal occurs.
- The length of the second group is 2, which is even, hence the nodes are reversed.
- The length of the third group is 3, which is odd, hence no reversal occurs.
- The length of the last group is 4, which is even, hence the nodes are reversed.
**Example 2:**
**Input:** head = [1,1,0,6]
**Output:** [1,0,1,6]
**Explanation:**
- The length of the first group is 1. No reversal occurs.
- The length of the second group is 2. The nodes are reversed.
- The length of the last group is 1. No reversal occurs.
**Example 3:**
**Input:** head = [1,1,0,6,5]
**Output:** [1,0,1,5,6]
**Explanation:**
- The length of the first group is 1. No reversal occurs.
- The length of the second group is 2. The nodes are reversed.
- The length of the last group is 2. The nodes are reversed.
**Constraints:**
* The number of nodes in the list is in the range [1, 105].
* 0 <= Node.val <= 105