解题思路：快慢指针法，慢指针走一步，快指针走两步，当快指针走到终点时，慢指针刚好指向中间节点

var midleNode =function(head){
    let fast =head
    let slot =head
    while(fast&&fast.next){
        slot=slot.next
        fast=fast.next.next
    }
    return slot
}