https://leetcode-cn.com/problems/linked-list-cycle-ii/

快慢指针

• 快指针一次走两步， 慢指针一次走一步

• 第一次相遇时，快指针回到原点，慢指针不动，接下来快指针也变成每次走一步，然后继续， 再次相遇就是第一个入环的节点

  public ListNode detectCycle(ListNode head) {
        if (head == null || head.next == null || head.next.next == null) {
            return null;
        }
        ListNode slow = head.next;
        ListNode fast = slow.next;
        while (slow != fast) {
            if (fast.next == null || fast.next.next == null) {
                return null;
            }
            fast = fast.next.next;
            slow = slow.next;
        }
        fast = head;
        while (slow != fast ) {
            fast = fast.next;
            slow = slow.next;
        }
        return slow;
    }