detectCycle

https://leetcode.com/problems/linked-list-cycle-ii/
给定一个链表，判断是否有环，并且返回环开始的那个ListNode

• 思路1
  HashSet存下来每一个走过的路，一直遍历，当发现set中有，则表明有环

    public ListNode detectCycle(ListNode head) {
      Set<ListNode> se = new HashSet<ListNode>();
        ListNode tmp = head;
        se.add(tmp);
        while (tmp != null && tmp.next != null) {
            tmp = tmp.next;
            if (se.contains(tmp)) {
                return tmp;
            }
            se.add(tmp);
        }
        return null;
    }

• 思路2
  1.快慢指针，先确定有换
  2.fast不变，把slow指针再从头开始，fast和slow同时走，再相交的地方就是环的起点

参考：https://www.cnblogs.com/grandyang/p/4137302.html

        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                break;
            }
        }
        if (fast == null || fast.next == null) {
            return null;
        }

        slow = head;
        while (slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }
        return fast;
    }