328.md

328. Odd Even Linked List

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 ...

Credits:
Special thanks to @DjangoUnchained for adding this problem and creating all test cases.

my thoughts:

1. two pointers.

my solution:

********** 96ms
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def oddEvenList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head or not head.next or not head.next.next:
            return head
        
        slow = head
        fast = head.next
        while fast and fast.next:
            temp = slow.next
            slow.next = fast.next
            fast.next = fast.next.next
            slow.next.next = temp
            slow = slow.next
            fast = fast.next
        return head

my comments:

from other ppl's solution:

1. faster, 56ms:
class Solution:
    def oddEvenList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head == None:
            return
        origeven = head.next
        
        odd = head
        even = head.next
        
        oddrunner = head
        evenrunner = even
        while oddrunner and evenrunner:
            oddrunner = oddrunner.next
            if oddrunner:
                oddrunner = oddrunner.next
            evenrunner = evenrunner.next
            if evenrunner:
                evenrunner = evenrunner.next
            
            odd.next = oddrunner
            if odd.next:
                odd = odd.next
                #print("odd")
                #print(odd.val)
            even.next = evenrunner
            if even.next:
                even = even.next
                #print("even")
                #print(even.val)
    
        odd.next = origeven
        return head