[Leetcode] 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?

On Cycle, the first idea is naturally done with HashMap. Note When using the Hashmap.put () method, value can be set to a random number.

Because we are only checking if key is duplicated, value is not very meaningful here.

The code is as follows. ~

/** * 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) {        hashmap<listnode,integer> hash=new Ha Shmap<listnode,integer> ();        if (head==null) {            return false;        }        while (head!=null) {            if (Hash.containskey (head)) {                return true;            }            Hash.put (head,1);            Head=head.next;        }        return false;    }}

But then look at follow up where the requirements are without extra space, so HashMap is temporarily unavailable.

You can do it with the pointer. Fast and Slow pointer. (Fase/slow) (fast every two steps, slow is a step.) ）

If there is a cycle, the fast and slow hands will meet.

/** * 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) {        if (head==null| | Head.next==null) {            return false;        }        ListNode Fast=head;        ListNode Slow=head;        while (fast!=null&&fast.next!=null) {            slow=slow.next;            Fast=fast.next.next;            if (slow==fast) {                return true;            }        }        return false;    }}

[Leetcode] Linked List Cycle