Finding cycle start in SLL is often presented wrong it its implementation, in fact even published. It is a simple mistake, but the correct version is a rare find as a result.

Find Cycle Start in SLL

//see https://stackoverflow.com/questions/2936213/explain-how-finding-cycle-start-node-in-cycle-linked-list-work/42615373#42615373
public Node whereLoopBegins(Node head) {
	Node slow = head;
	Node fast = head;
	// collide them at loopsize - K , K = k % loopsize

	while (fast != null && fast.next != null && fast.next.next != null) {
	    slow = slow.next;
	    fast = fast.next.next;
	    if (slow == fast) { // there is a cycle, but slow and fast may have
				// met at head due to a 0 length start to loop

		int cLength = 0;
		do {
		    ++cLength;
		} while (slow != fast);

		fast = head;
		while (cLength-- > 0) {
		    fast = fast.next;
		}
		slow = head;
		while (fast != slow) {
		    fast = fast.next;
		    slow = slow.next;
		}
		return slow;
	    }
	}
	return null;
}