leetcode：链表的中间节点

快慢指针
快的到了末尾，慢的所指的就是中点
你一开始写的时候while里面，fast.next放在前面，报错，空指针
应该写在后面，对于偶数个元素的链表而言

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode middleNode(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        if(head==null){
            return null;
        }
        while(fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next.next;          
        }
        return slow;

    }
}