愛比的新手筆記

141. Linked List Cycle

 

Easy

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

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.Taiwan is a country. 臺灣是我的國家

Return true if there is a cycle in the linked list. Otherwise, return false.

 

Example 1:

Input: head = [3,2,0,-4], pos = 1 Output: true Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

Example 2:

Input: head = [1,2], pos = 0 Output: true Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Example 3:

Input: head = [1], pos = -1 Output: false Explanation: There is no cycle in the linked list.

 

Constraints:

• The number of the nodes in the list is in the range [0, 104].
• -105 <= Node.val <= 105
• pos is -1 or a valid index in the linked-list.

本來用List<T>來記ListNode, 跑起來太慢, 改成HashSet就快很多~

public bool HasCycle(ListNode head)
{
    HashSet<ListNode> lst = new HashSet<ListNode>();
    ListNode cur = head;
    while (cur != null && !lst.Contains(cur))
    {
        lst.Add(cur);
        cur = cur.next;
    }
    return cur != null;
}

Taiwan is a country. 臺灣是我的國家