public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head==null) return false;
        ListNode curr=head, ahead=head;
        
        while(ahead.next!=null&&ahead.next.next!=null){
            curr=curr.next;
            ahead=ahead.next.next;
            
            if(curr==ahead)
                return true;
        }
        
        return false;
    }
}