328. Odd Even Linked List

这里的even和odd指的是index。奇数位的连成一个链表，偶数位的连成一个链表，最后这两个链表连一连。

public ListNode oddEvenList(ListNode head) {
    if (head == null) return head;
    ListNode odd = head,
            even = head.next,
            evenHead = even;

    while (even != null && even.next != null) {
        odd.next = odd.next.next;
        even.next = even.next.next;
        odd = odd.next;
        even = even.next;
    }
    // concat
    odd.next = evenHead;
    return head;
}