328. Odd Even Linked List (Medium)

https://leetcode.com/problems/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

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 ...

Solutions

class Solution {

    public ListNode oddEvenList(ListNode head) {
        // return head if head == null or head.next == null or hext.next.next == null
        if (head == null || head.next == null || head.next.next == null) {
            return head;
        }

        // A->B->C->D

        // p1 links to A, p2 links B, p3 links C
        ListNode p1 = head;
        ListNode p2 = p1.next;
        ListNode p3 = p2.next;

        // stop traverse list if p3 == null
        while (p3 != null) {
            // A->B->D C->D
            // p1->p2->D p3->d
            p2.next = p3.next;

            // A->B->D C->B->D
            p3.next = p1.next;

            // A->C->B->D
            p1.next = p3;

            // p1->p3->p2->D

            p1 = p1.next;
            p2 = p2.next;

            // break if p2 == null, D may be null
            if (p2 == null) {
                break;
            }

            // p1->C->B->p2->p3
            p3 = p2.next;
        }

        return head;
    }
}

Incorrect Solutions

References