题解 | #判断链表中是否有环#

public class Solution {
    public boolean hasCycle(ListNode head) {
        //先判断链表为空的情况 fast-template
        if (head == null)
            return false;
        //快慢双指针
        ListNode fast = head;
        ListNode slow = head;
        //如果没环快指针会先到链表尾
        while (fast != null && fast.next != null) {
            //快指针移动两步
            fast = fast.next.next;
            //慢指针移动一步
            slow = slow.next;
            //相遇则有环
            if (fast == slow)
                return true;
        }
        //到末尾则没有环
        return false;
    }
}