环形链表1：

设置两个指针，
慢指针一次走一步，快指针一次走两步，
如果 fast == null 或者 fast.next == null 不存在环，
如果存在环，两个指针进入环中，是一个追及问题，一定会相遇

var hasCycle = function (head) {if (head == null || head.next == null) {return false}let slow = head;let fast = head;while (fast !== null && fast.next !== null) {slow = slow.nextfast = fast.next.nextif (slow === fast) {return true}}return false
};

环形链表2：
在环形链表1的基础上，先判断是否有环
如果有环，快慢指针相遇时，将快指针指向head，然后每次走一步
当快慢指针再次相遇时，返回这个ListNode

var detectCycle = function (head) {if (head == null || head.next == null) {return null}let slow = head;let fast = head;let isCycle = false;while (fast != null && fast.next != null) {slow = slow.nextfast = fast.next.nextif (slow == fast) {isCycle = truefast = headbreak}}if (!isCycle) {return null} else {while (fast != null && fast.next != null) {if (slow === fast) {return fast}fast = fast.nextslow = slow.next}}
};