Ruby

1# Definition for singly-linked list.
2# class ListNode
3#     attr_accessor :val, :next
4#     def initialize(val = 0, _next = nil)
5#         @val = val
6#         @next = _next
7#     end
8# end
9# @param {ListNode} head
10# @return {ListNode}
11def odd_even_list(head)
12    return head if !head or !head.next or !head.next.next
13    
14    odd = head
15    even = head.next
16    temp = even
17    prev = nil
18    
19    while odd && odd.next
20        odd.next = odd.next.next if odd && odd.next
21        prev = odd
22        odd = odd.next
23        temp.next = temp.next.next if temp && temp.next
24        temp = temp.next
25    end 
26    
27    if odd == nil
28        prev.next = even
29    else
30        odd.next = even
31    end
32    
33    head
34end