
Contents
For Solution
Click Here!
You are given the head
of a linked list.
The nodes in the linked list are sequentially assigned to nonempty 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
1^{st}
node is assigned to the first group.  The
2^{nd}
and the3^{rd}
nodes are assigned to the second group.  The
4^{th}
,5^{th}
, and6^{th}
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 occurrs.  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 occurrs.  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 occurrs.  The length of the second group is 2. The nodes are reversed.  The length of the last group is 1. No reversal occurrs.
Example 3:
Input: head = [2,1] Output: [2,1] Explanation:  The length of the first group is 1. No reversal occurrs.  The length of the last group is 1. No reversal occurrs.
Example 4:
Input: head = [8] Output: [8] Explanation: There is only one group whose length is 1. No reversal occurrs.
Constraints:
 The number of nodes in the list is in the range
[1, 10^{5}]
. 0 <= Node.val <= 10^{5}

For Solution
Click Here!