141. 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?

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