LeetCode 141 链表 . Linked List Cycle

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

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example：

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Follow up：
Can you solve it using O(1) (i.e. constant) memory?

代码：

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

注意：
快指针时刻注意空指针错误