/** * Definition for singly-linked list. * function ListNode(val) { * this.val = val; * this.next = null; * } */ /** * @param {ListNode} head * @return {ListNode} */ var detectCycle = function(head) { if (!head || !head.next) return null; var slow = head; var fast = head; while (slow.next && fast.next && fast.next.next) { slow = slow.next; fast = fast.next.next; if (slow === fast) { slow = head; while (slow !== fast) { slow = slow.next; fast = fast.next; } return slow; } } return null; };