題目
Given a non-empty, singly linked list with head node head, return a middle node of linked list.

If there are two middle nodes, return the second middle node.

Example 1:

Input: [1,2,3,4,5]
Output: Node 3 from this list (Serialization: [3,4,5])
The returned node has value 3. (The judge's serialization of this node is [3,4,5]).
Note that we returned a ListNode object ans, such that:
ans.val = 3, ans.next.val = 4, ans.next.next.val = 5, and ans.next.next.next = NULL.
Example 2:

Input: [1,2,3,4,5,6]
Output: Node 4 from this list (Serialization: [4,5,6])
Since the list has two middle nodes with values 3 and 4, we return the second one.

答案

class Solution {
    public ListNode middleNode(ListNode head) {
        if(head == null) return null;
        ListNode fast = head, slow = head;
        while(slow.next != null) {
            if(slow.next.next == null) return fast.next;
            slow = slow.next.next;
            fast = fast.next;
        }
        return fast;
    }
}