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?

 

思路：模拟两个人沿着固定路线跑步，看先跑的人是否可以追上后跑的人

java代码：

1. public boolean hasCycle(ListNode head) {
2. if(head==null || head.next==null) return false;
3. ListNode slow=head;
4. ListNode fast = head;
5. while(fast!=null && fast.next!=null) {  //注意判断边界条件
6. slow=slow.next;
7. fast=fast.next.next;
8. // if(fast==null) return false;
9. if(fast==slow) return true;
10. }
11. return false;
12. }