141. Linked List Cycle

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