328. Odd Even Linked List——多利用fake_head

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:
Given 1->2->3->4->5->NULL,
return 1->3->5->2->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(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def oddEvenList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        node = head
        i = 1
        odd_fake, even_fake = ListNode(None), ListNode(None)
        odd_tail, even_tail = odd_fake, even_fake
        while node:
            if i & 1:
                odd_tail.next = node
                odd_tail = node
            else:
                even_tail.next = node
                even_tail = node
            node = node.next
            i += 1
        even_tail.next = None
        odd_tail.next = even_fake.next
        return odd_fake.next