141.Linked List Cycle === ###### tags: `Easy` `Hash Table` `Linked List` `Two Pointers` [141. Linked List Cycle](https://leetcode.com/problems/linked-list-cycle/) ### 題目描述 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**. Return `true` *if there is a cycle in the linked list.* Otherwise, return `false`. ### 範例 **Example 1:** ![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png) ``` 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:** ![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png) ``` 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:** ![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png) ``` 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, 10^4^]. * -10^5^ <= `Node.val` <= 10^5^ * `pos` is `-1` or a **valid index** in the linked-list. ### 解答 ### Reference [回到題目列表](https://marsgoat.github.io/XNnote/coding/leetcodeEveryDay.html)