LeetCode-Easy刷题(32) 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?

给定一个链表，确定它是否有一个循环。  你能在不使用额外空间的情况下解决它吗?

 //两个速度指针    public boolean hasCycle(ListNode head) {       if(head ==null){           return false;       }       ListNode walker = head;       ListNode runner = head;       while(runner!=null && runner.next!=null){           walker = walker.next;           runner = runner.next.next;           if(walker == runner){               return true;           }       }       return false;    }