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
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