/**
 * 各种求链表中间节点
 */
public class FindMidNode {

    /**
     * 输入链表头结点，奇数长度返回中点，偶数长度返回上中点
     *
     * @param head 头结点
     * @return 中点或者上中点
     */
    public Node midOrUpMidNode(Node head) {
        if (head == null || head.next == null || head.next.next == null) {
            return head;
        }
        Node slow = head.next;
        Node fast = head.next.next;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    /**
     * 输入链表头结点，奇数长度返回中点，偶数长度返回下中点
     *
     * @param head 头结点
     * @return 中点或者下中点
     */
    public Node midOrDownMidNode(Node head) {
        if (head == null || head.next == null) {
            return head;
        }
        Node slow = head.next;
        Node fast = head.next;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    /**
     * 输入链表头结点，奇数长度返回中点前一个，偶数长度返回上中点前一个
     *
     * @param head 头结点
     * @return 结点
     */
    public Node midOrUpMidPreNode(Node head) {
        if (head == null || head.next == null || head.next.next == null) {
            return null;
        }
        Node slow = head;
        Node fast = head.next.next;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    /**
     * 输入链表头结点，奇数长度返回中点前一个，偶数长度返回下中点前一个
     *
     * @param head 头结点
     * @return 结点
     */
    public Node midOrDownMidPreNode(Node head) {
        if (head == null || head.next == null) {
            return null;
        }
        if (head.next.next == null) {
            return head;
        }
        Node slow = head;
        Node fast = head.next;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    /**
     * 链表结构
     */
    public static class Node {

        public int value;

        public Node next;

        public Node(int value) {
            this.value = value;
        }

    }

}

/* 如有意见或建议，欢迎评论区留言；如发现代码有误，欢迎批评指正 */