141. 环形链表

哈希表

遍历将结点存储在哈希表中，如果遇到环的入口，会检测到

public class Solution {
    public boolean hasCycle(ListNode head) {
        Set<ListNode> seen = new HashSet<ListNode>();
        while (head != null) {
            if (!seen.add(head)) {
                return true;
            }
            head = head.next;
        }
        return false;
    }
}

快慢指针

public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null) {
            return false;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while (slow != fast) {
            if (fast == null || fast.next == null) {
                return false;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        return true;
    }
}