/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func middleNode(head *ListNode) *ListNode {
    // v1
    // 1->2->3->4->5->nil
    // 1->2->3->4->5->6->nil
    if head==nil{
        return nil
    }
    fast := head
    slow := head
    for fast != nil && fast.Next != nil {
        fast = fast.Next.Next
        slow = slow.Next
    }
    return slow
}
func middleNode2(head *ListNode) *ListNode {
    // v2分为两种情况：
    // 1->2->3->4->5->nil
    // 1->2->3->4->5->6->nil
    if head==nil{
        return nil
    }
    fast := head.Next
    slow := head
    for fast != nil && fast.Next != nil {
        fast = fast.Next.Next
        slow = slow.Next
    }
    if fast==nil{
        return slow
    }
    return slow.Next
}