OddEvenLinkedList.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class OddEvenLinkedList {
    public ListNode oddEvenList(ListNode head) {
                if(head == null || head.next == null)return head;

        ListNode odd = head;
        ListNode evenHead = head.next;
        ListNode even = evenHead;

        //while (even!=null && even.next != null){
        while(even!=null && even.next!=null){
            odd.next = odd.next.next;
            odd = odd.next;
            even.next = even.next.next;
            even = even.next;
        }

        odd.next = evenHead;
        return head;
    }
}