https://leetcode.com/problems/linked-list-cycle-ii/#/description
Given a linked list, return the node where the cycle begins. If there is no cycle, return null. Note: Do not modify the linked list. Follow up: Can you solve it without using extra space?
public ListNode detectCycle(ListNode head) { if (head == null) { return null; } ListNode fast = head; ListNode slow = head; do { if (fast == null || fast.next == null) { return null; } fast = fast.next.next; slow = slow.next; } while (fast != slow); fast = head; while (fast != slow) { fast = fast.next; slow = slow.next; } return fast; }