Linked List Cycle

Linked List Cycle

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Given a linked list, determine if it has a cycle in it.
Follow up:

Can you solve it without using extra space?
/**
* 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, fast = head;
while(fast!=null && fast.next!=null){
slow = slow.next;
fast = fast.next.next;
if(slow == fast) return true;
}
return false;
}
}