LeetCode141-环形链表

哈希表

//哈希表
public boolean hasCycle1(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

//快慢指针
public boolean hasCycle(ListNode head) {
    if (head == null || head.next == null) return false;
    ListNode k = head.next;
    ListNode m = head;
    while (k != null && k.next != null) {
        if (k == m) return true;
        m = m.next;
        k = k.next.next;
    }
    return false;
}