May Leetcode Challenge - Day 16

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 1:

Input: 1->2->3->4->5->NULL
Output: 1->3->5->2->4->NULL

Example 2:

Input: 2->1->3->5->6->4->7->NULL
Output: 2->3->6->7->1->5->4->NULL


  • 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 ...


Fast and slow technique

class Solution {
    func oddEvenList(_ head: ListNode?) -> ListNode? {
        var slow = head
        var fast = head?.next
        var evenRoot = head?.next
        while fast != nil, fast?.next != nil {
            slow?.next = fast?.next
            slow = fast?.next
            fast?.next = slow?.next
            fast = slow?.next
        slow?.next = evenRoot
        return head