剑指offer-23-链表中环的入口节点-golang

题目：如果一个链表中包含环，如何找出环的入口节点。

type Node struct {
	val  int
	next *Node
}

func hasCir(head *Node) (*Node, bool) {
	if head == nil || head.next == nil || head.next.next == nil {
		return nil, false
	}
	front, tail := head.next, head
	for front != nil && front.next != nil && front != tail {
		front = front.next.next
		tail = tail.next
	}
	if front == tail {
		return front, true
	}
	return nil, false
}

func NodeNumCir(head *Node) int {
	p, hCir := hasCir(head)
	if !hCir {
		return 0
	}
	pm := p.next
	count := 1
	for p != pm {
		count++
		pm = pm.next
	}
	return count
}

func CirFirstNode(head *Node) int {
	if head == nil {
		return 0
	}
	nodeCount := NodeNumCir(head)
	if nodeCount < 1 {
		return 0
	}
	front, tail := head, head
	for i := 0; i < nodeCount; i++ {
		front = front.next
	}
	for front != tail {
		front = front.next
		tail = tail.next
	}
	return front.val
}