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

Follow up:
Can you solve it without using extra space?

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null) return false;
        ListNode n1 = head, n2 = head;
        while(n2.next != null && n2.next.next != null) {
            n1 = n1.next;
            n2 = n2.next.next;
            if(n1 == n2) return true;
        }
        return false;
    }
}