（链表）06-判断链表中是否有环

/**
 * 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) {
        // 定义临时变量-快慢两个指针
        ListNode fast = head;
        ListNode slow = head;
        // 循环链表
        while(fast != null && fast.next != null) {
            // 快指针走两步
            fast = fast.next.next;
            // 慢指针走一步
            slow = slow.next;
            // 快慢指针相遇则说明有环
            if (fast == slow) {
                return true;
            }
        }
        // 链表遍历到尾节点说明没有环
        return false;
    }
}