题解 | #重排链表#

    public void reorderList(ListNode head) {
        if (head == null || head.next == null || head.next.next == null) {
        return;
    }
    //找中点，链表分成两个
    ListNode slow = head;
    ListNode fast = head;
    while (fast.next != null && fast.next.next != null) {
        slow = slow.next;
        fast = fast.next.next;
    }

    ListNode newHead = slow.next;
    slow.next = null;
    
    //第二个链表倒置，反转后半部分链表
    newHead = reverseList(newHead);
    
    //链表节点依次连接
    while (newHead != null) {
        ListNode temp = newHead.next;
        newHead.next = head.next;
        head.next = newHead;
        head = newHead.next;
        newHead = temp;
    }
}

    private ListNode reverseList(ListNode head) {
        if (head == null) {
            return null;
        }
        ListNode tail = head;
        head = head.next;
        tail.next = null;
        while (head != null) {
            ListNode temp = head.next;
            head.next = tail;
            tail = head;
            head = temp;
        }
        return tail;
    }
}