"""Given a singly linked list, group all odd nodes together followed by the even nodes. Please note here we are talking about the node number and not the value in the nodes.

You should try to do it in place. The program should run in O(1) space complexity and O(nodes) time complexity.

Example 1:

Input: 1->2->3->4->5->NULL
Output: 1->3->5->2->4->NULL
Example 2:

Input: 2->1->3->5->6->4->7->NULL
Output: 2->3->6->7->1->5->4->NULL


Constraints:

The relative order inside both the even and odd groups should remain as it was in the input.
The first node is considered odd, the second node even and so on ...
The length of the linked list is between [0, 10^4]."""


# Definition for singly-linked list.
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

class Solution:
    def oddEvenList(self, head: ListNode) -> ListNode:
        se = ListNode()
        se.next = head
        odd = None
        even = None
        even_head = None

        if se.next is not None:
            odd = se.next
        else:
            return head

        if se.next.next is not None:
            even = se.next.next
            even_head = se.next.next
        else:
            return head

        while odd and odd.next and even and even.next:
            temp_even = None
            if odd.next.next is None:
                temp_even = odd.next
                odd.next = None
                even.next = temp_even
                break
            else:
                temp_even = odd.next
                odd.next = odd.next.next
                odd = odd.next
                even.next = temp_even
                even = even.next
        else:
            odd.next = None
            even.next = None

        odd.next = even_head

        return head

# Submission Details:
# Runtime: Faster than 95.66% python3 submissions
# Memory: Better than 87.46% python3 submissions
