Java 8

    static boolean hasCycle(SinglyLinkedListNode head) {
        Set<SinglyLinkedListNode> nodeSet = new HashSet<>();
        SinglyLinkedListNode node = head;
        while (node != null) {
            if (nodeSet.contains(node)) {
                return true;
            }
            nodeSet.add(node);
            node = node.next;
        }
        return false;
    }

This question does have bugs in some languages, such as JavaScript. I ran the following which is exactly equivalent to the working Python 3 solution already posted. That solution passes in Python, but it does not pass in JavaScrpt.

function hasCycle(head) { let node = head; const visited = {}; while (node) { if (visited[node]) return true; visited[node] = true; node = node.next; } return false; }

again this question is bugged... the example of the cycle:

1 -> 2 -> 3 -> 1 -> NULL

1 can't point to both 1 and NULL!

At least this time the ro class compiles!

static bool hasCycle(SinglyLinkedListNode head) {
    var d = new HashSet<SinglyLinkedListNode>();
    while(head is not null){
        if(d.Contains(head)) return true;
        d.Add(head);
        head = head.next;
    }
    return false;
}

Only python works for me...

Floyd cycle approach

def has_cycle(head):
    slow = head
    fast = head
    
    while fast is not None and fast.next is not None:
        slow = slow.next
        fast = fast.next.next
        
        if (slow == fast):
            return 1
    
    return 0

static boolean hasCycle(SinglyLinkedListNode head) { if (head == null || head.next == null) { return false; } SinglyLinkedListNode slow = head; SinglyLinkedListNode fast = head.next; while (fast != null && fast.next != null) { if (slow == fast) { return true; } slow = slow.next; fast = fast.next.next; } return false; }