Linked List Cycle——LeetCode

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

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

题目大意：给定一个链表，判断是否有环？

解题思路：

解法一：快慢指针，如果有环，那么快慢指针总会相遇，有环；否则快的遍历完整个链表，无环。

解法二：HashSet保存节点，如果下一个节点在set中出现过，那么有环，否则直到遍历完整个链表，无环。

Talk is cheap>>

解法一：

    public boolean hasCycle(ListNode head) {

        if (head == null || head.next == null) {

            return false;

        }

        ListNode slow = head;

        ListNode fast = head;

        while (fast.next != null && fast.next.next != null) {

            fast = fast.next.next;

            slow = slow.next;

            if (slow == fast) {

                return true;

            }

        }

        return false;

    }

解法二：

    public boolean hasCycle2(ListNode head) {

        if (head == null || head.next == null) {

            return false;

        }

        Set<ListNode> set = new HashSet<>();

        while (head != null) {

            if (set.contains(head)) {

                return true;

            }

            set.add(head);

            head = head.next;

        }

        return false;

    }