OddEvenList.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */

/*
Runtime: 0 ms, faster than 100.00% of Java online submissions for Odd Even Linked List.
Memory Usage: 38.6 MB, less than 59.39% of Java online submissions for Odd Even Linked List.
*/
class OddEvenList {
    public ListNode oddEvenList(ListNode head) {
     if (head == null) return null;
        ListNode odd = head, even = head.next, evenHead = even;
        while (even != null && even.next != null) {
            odd.next = even.next;
            odd = odd.next;
            even.next = odd.next;
            even = even.next;
        }
        odd.next = evenHead;
        return head;
    }
}