141.环形链表

/**

• Definition for singly-linked list.
• type ListNode struct {

• Val int
  

• Next *ListNode
  

• }
  */
  func hasCycle(head *ListNode) bool {
  hash := make(map[*ListNode]int)
  for head !=nil{
  if _,ok :=hash[head];ok{
  return true
  }
  hash[head]=head.Val
  head = head.Next
  }
  return false
  }