package q142

type ListNode struct {
	Val  int
	Next *ListNode
}

func detectCycle(head *ListNode) *ListNode {
	p1, p2 := head, head
	for p1 != nil && p2 != nil {
		p1 = p1.Next
		p2 = p2.Next
		if p2 != nil {
			p2 = p2.Next
		}
		if p1 == p2 {
			break
		}
	}
	if p2 == nil {
		return nil
	}

	p1 = head
	for p1 != p2 {
		p1 = p1.Next
		p2 = p2.Next.Next
	}
	return p1
}

var _ = detectCycle