LC 141. Linked List Cycle.txt

/**
 * 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) {
        ListNode slow = head;
        ListNode fast = head;

        do
        {
            if( slow == null || fast == null || fast.next == null ) return false;
            slow = slow.next;
            fast = fast.next.next;
        }while( slow != fast );
        return true;
    }
}
// TC: O(n), SC: O(1)
// Can also be solved using a HashSet<ListNode> with the TC: O(n), SC: O(n)