package main

type ListNode struct {
	Val  int
	Next *ListNode
}

func main() {

}
func detectCycle(head *ListNode) *ListNode {
	hash := make(map[*ListNode]struct{})
	for head != nil {
		if _, ok := hash[head.Next]; ok {
			return head.Next
		}
		hash[head] = struct{}{}
		head = head.Next
	}

	return nil
}