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 <code>1^st</code> node is assigned to the first group.

• The <code>2^nd</code> and the <code>3^rd</code> nodes are assigned to the second group.

• The <code>4^th</code>, <code>5^th</code>, and <code>6^th</code> 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 <code>1, 10⁵</code>.

• <code>0 <= Node.val <= 10⁵</code>