链表中间数

示例：
输入：[1,2,3,4,5]
输出：此列表中的结点 3 (序列化形式：[3,4,5])

输入：[1,2,3,4,5,6]
输出：此列表中的结点 4 (序列化形式：[4,5,6])

1、遍历链表，新建一个ListNode数组，将链表元素copy到数组，数组的length/2下标就是目标Node
  public static ListNode middleNode(ListNode head) {
        ListNode[] ints = new ListNode[countLength(head)];

        int i = 0;
        ints[0] = head;
        while (head.next != null) {
            i += 1;
            ints[i] = head.next;
            head = head.next;
        }
        return ints[ints.length / 2];
    }

    public static int countLength(ListNode head) {
        int length = 1;
        while (head.next != null) {
            length += 1;
            head = head.next;
        }
        return length;
    }

    static class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
        }
    }
2、快慢指针，慢指针依次遍历，快指针比慢指针快两倍
public static ListNode middleNode2(ListNode head) {
        ListNode p = head, q = head;
        while (p != null && q.next != null) {
            p = p.next;
            q = q.next.next;
        }
        return p;
    }