141. Linked List Cycle

public boolean hasCycle(ListNode head) {
if(head == null) {
return false;
}
ListNode runner = head;
ListNode walker = head;
while(runner.next != null && runner.next.next != null) {
walker = walker.next;
runner = runner.next.next;
if(walker == runner) {
return true;
}
}
return false;
}