环形链表

给出一个链表,若其中包含环，请找出该链表的环的入口节点，否则输出null

var findNode = (head)=>{
    if(head.next == null || head == null){
        return null
    }
    var slow = head
    var fast = head
    while(fast != slow){
        if(fast == null || fast.next == null){
            return null
        }
        fast = fast.next.next
        slow = slow.next
    }
    // 环的长度
    let length = 1
    let temp = slow
    slow = slow.next
    if(slow != temp){
        length++
    }
    // 找到环的位置
    fast =slow = head
    while(length-->0){
        fast = fast.next
    }
    while(fast == slow){
        slow = slow.next
        fast = fast.next
    }
    return slow
}