Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Follow up:

Can you solve it without using extra space?

定义两个指针，一个快，一个慢，快的追到慢的则存在环。

public Boolean hasCycle(ListNode head) {if (head == null) return false;ListNode fast, slow;fast = head.next;slow = head;while (fast != slow) {if(fast==null || fast.next==null)return false;fast = fast.next.next;slow = slow.next;} return true;}