143. Reorder List 

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class Solution {
    public void reorderList(ListNode head) {
        ListNode dummyHead = new ListNode(-1, head);
        ListNode fast = dummyHead, slow = dummyHead;

        // d -> 1 -> 2 -> 3 -> 4
        // f
        // s
        //           f
        //      s
        //                    f
        //           s

        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }

        ListNode rightHead = slow.next;
        slow.next = null;

        rightHead = reverse(rightHead);
        ListNode nodeA = head, nodeB = rightHead;
        while (nodeB != null) {
            ListNode nextA = nodeA.next;
            ListNode nextB = nodeB.next;

            nodeA.next = nodeB;
            nodeB.next = nextA;

            nodeA = nextA;
            nodeB = nextB;
        }
    }

    private ListNode reverse(ListNode head) {
        ListNode pre = null, curr = head;

        while (curr != null) {
            ListNode next = curr.next;
            curr.next = pre;
            pre = curr;
            curr = next;
        }

        return pre;
    }
}

References

143. Reorder List
剑指 Offer II 026. 重排链表