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;
    }