[LintCode] Linked List Cycle

public boolean hasCycle(ListNode head) {  
        // write your code here
        if(head == null)
            return false;
        ListNode fast = head;
        ListNode slow = head;
        while(fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if(slow == fast)
                return true;
        }
        return false;
    }