快慢指针

奇数返回中点，偶数返回上中点

slow=head.next;
fast=head.next.next;
while(head!=null){
if(head==null||head.next=null||head.next.next==null;){
return head;}
if(fast.next!=null&&fast.next.next!=null){
	slow=slow.next;
	fast=fast.next.next;
}
return slow;
}

奇数返回中点，偶数返回下中点

slow=head.next;
fast=head.next;
while(head!=null){
if(head==null||head.next=null||head.next.next==null;){
return head;}
if(fast.next!=null&&fast.next.next!=null){
	slow=slow.next;
	fast=fast.next.next;
}
return slow;
}

奇数返回中点前一个，偶数返回上中点前一个

slow=head;
fast=head.next.next;
while(head!=null){
if(head==null||head.next=null||head.next.next==null;){
return null;}
if(fast.next!=null&&fast.next.next!=null){
	slow=slow.next;
	fast=fast.next.next;
}
return slow;
}

奇数返回中点前一个，偶数返回下中点前一个

slow=head;
fast=head.next;
while(head!=null){
if(head==null||head.next=null){
return null;}
if(head.next.next==null;){
return head;}
if(fast.next!=null&&fast.next.next!=null){
	slow=slow.next;
	fast=fast.next.next;
}
return slow;
}