"Leetcode" Linked List Cycle

First, the original question

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?

Second, analysis

Choose two pointers to scan the linked list, a fast speed, a slow, if two pointers meet, indicating a ring.

Third, code (Java)

/** * Definition for singly-linked list.  * Class ListNode {*     int val; *     ListNode Next; *     listnode (int x) {*         val = x; *         next = null; *     } * } */public Class Solution {public    Boolean hascycle (ListNode head) {       boolean result=false;if (head==null| | Head.next==null) {}else{    listnode fast,slow;        Slow=head;    Fast=head;    while (Fast!=null)   {  if (fast.next==null| | Fast==null) break  ;  else{  Fast=fast.next.next;      Slow=slow.next;      }  if (fast==slow) {result=true; break;}}   } return result;}    }

Four

About the speed of the meeting indicates the link list has a ring proof:

http://umairsaeed.com/2011/06/23/finding-the-start-of-a-loop-in-a-circular-linked-list/.

More intuitive explanation, if two points in a circle running, the same direction, as long as the speed is not the same will meet, in the straight line is not necessarily.

"Leetcode" Linked List Cycle