Linked List Cycle

Given a linked list, determine if it has a cycle in it.

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head==null) return false;
        ListNode fast = head;
        ListNode slow = head;
        while(fast!=null){
            slow = slow.next;
            fast = fast.next;
            if(fast==null)
                return false;
            fast = fast.next;
            if(fast==slow) return true;
        }
        return false;
    }
}