Linked list cycle

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

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

快慢指针，有环的话总会相遇。while的条件还是指针是否Null

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null || head.next == null){
            return false;
        }
        ListNode slow = head, quick = head;
        while(quick != null && quick.next != null){
            slow = slow.next;
            quick = quick.next.next;
            if(slow == quick){
                return true;
            }
        }
        return false;
    }
}