LeetCode 328. Odd Even Linked List

Description

https://leetcode.com/problems/odd-even-linked-list/

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.

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

Follow up: Could you solve it in O(1) space complexity and O(nodes) time complexity?

Explanation

Create an odd list and an even list, then combine together.

Python Solution

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def oddEvenList(self, head: ListNode) -> ListNode:

        odd_head = ListNode(0)
        odd = odd_head
        
        even_head = ListNode(0)
        even = even_head
        
        count = 0
        while head != None:
            if count % 2 == 0:
                odd.next = head
                odd = head
            else:
                even.next = head
                even = head
            
            count += 1
            head = head.next
        
        even.next = None
        odd.next = even_head.next
        
        
        return odd_head.next

• Time complexity: O(N).
• Space complexity: O(1).