/*
 * @lc app=leetcode.cn id=142 lang=golang
 *
 * [142] 环形链表 II
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func detectCycle(head *ListNode) *ListNode {
	if head == nil || head.Next == nil {
		return nil
	}
	slow := head.Next
	fast := head.Next.Next
	for slow != fast {
		if fast == nil || fast.Next == nil {
			return nil
		}
		slow = slow.Next
		fast = fast.Next.Next
	}
	// 到这了，只能证明有环，并不知道快慢指针在哪里相遇
	p := head
	for p != slow {
		p = p.Next
		slow = slow.Next
	}
	return p
}

// @lc code=end