old_contents: """
  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
  Note:
  
  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 ...
  """
  
  # Definition for singly-linked list.
  # class ListNode:
  #     def __init__(self, x):
  #         self.val = x
  #         self.next = None
  
  class Solution:
      def oddEvenList(self, head: ListNode) -> ListNode:
          if not head:
              return None
  
          odd_current = head
          even_current = head.next
          even_head = head.next
          current = head
          i = 0
          while current:
              if i % 2 == 0:
                  even_current.next = current
                  even_current = even_current.next
              else:
                  odd_current.next = current
                  odd_current = odd_current.next
  
              i += 1
              current = current.next
  
  
  
  
          return head
  
new_contents: """
  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
  Note:
  
  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 ...
  """
  
  # Definition for singly-linked list.
  # class ListNode:
  #     def __init__(self, x):
  #         self.val = x
  #         self.next = None
  
  class Solution:
      def oddEvenList(self, head: ListNode) -> ListNode:
          if not head:
              return None
  
          odd_dummy = ListNode(0)
          even_dummy = ListNode(0)
          cur_odd = odd_dummy
          cur_even = even_dummy
          is_even = False
          while head:
              cur_odd.next = head
              cur_even.next = head.next
              cur_odd = cur_odd.next
              cur_even = cur_even.next
              if head.next:
                  head = head.next.next
              else:
                  head = None
  
          cur_odd.next = even_dummy.next
  
          return odd_dummy.next
  
subject: Update 328