328. Odd Even Linked List

[DNPotapov]

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def oddEvenList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        res = ListNode(None)
        save = ListNode(None)
        cur = head
        s1 = res
        s2 = save
        count = 1
        while cur:
            if count%2 == 0:
                s2.next = ListNode(cur.val)
                s2 = s2.next
            else:
                s1.next = ListNode(cur.val)
                s1 = s1.next
            cur = cur.next
            count += 1
        s1.next = save.next
        return res.next

[DNPotapov]

Given the head of a singly linked list, group all the nodes with odd indices together followed by the nodes with even indices, and return the reordered list.

The first node is considered odd, and the second node is even, and so on.

Note that the relative order inside both the even and odd groups should remain as it was in the input.

You must solve the problem in O(1) extra space complexity and O(n) time complexity.

Example 1:

Input: head = [1,2,3,4,5] Output: [1,3,5,2,4] Example 2:

Input: head = [2,1,3,5,6,4,7] Output: [2,3,6,7,1,5,4]

Constraints:

The number of nodes in the linked list is in the range [0, 104]. -106 <= Node.val <= 106