Example 1:

Input: head = [3,2,0,-4], pos = 1

Output: true

Explanation: There is a cycle in the linked list, where tail connects to the second node.

Example 2:

Input: head = [1,2], pos = 0

Output: true

Explanation: There is a cycle in the linked list, where tail connects to the first node.

Example 3:

Input: head = [1], pos = -1

Output: false

Explanation: There is no cycle in the linked list.

class Solution {

public:

    bool hasCycle(ListNode *head) {

        ListNode *slow = head, *fast = head;

        while (fast && fast->next) {

            slow = slow->next;

            fast = fast->next->next;

            if (slow == fast) return true;

        }

        return false;

    }

};

public class Solution {

    public boolean hasCycle(ListNode head) {

        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;

    }

}