leetcode-141. 环形链表

/**
 * 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) {
        // 判空
        if (head == null || head.next == null) {
            return false;
        }
        // 定义一个慢指针
        ListNode slow = head;
        // 定义一个快指针
        ListNode fast = head.next;
        // 当慢指针指向的节点等于快指针指向的节点则存在环
        while (slow != fast) {
            if (fast == null || fast.next == null) {
                return false;
            }
            // 慢指针走一步
            slow = slow.next;
            // 快指针走两步
            fast = fast.next.next;
        }
        return true;
    }
}

/**
 * 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) {
        // 利用 set 的不可重复的特性
        Set<ListNode> seen = new HashSet<ListNode>();
        // 将所有的节点存到 set 里边，若存不进去则存在环
        while (head != null) {
            if (!seen.add(head)) {
                return true;
            }
            head = head.next;
        }
        return false;
    }
}