Given a linked list, determine if it has a cycle in it.
To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.
Follow up:
Can you solve it using O(1) (i.e. constant) memory?
方法一：双指针
1、一个指针移动一步，一个指针移动两步，如果两个指针相遇则有环。
2、做任何题，第一个应该想的是鲁棒性
算法复杂度：o(n) 运行时间：0ms 占用空间：38.6ms
方法二：哈希表
1、什么时候想到用哈希表？如果要查找一个元素是否在一个集合中。常常想到哈希表。如果用键值对用hashmap，否则用hashset。
算法复杂度：o(n) 运行时间：5ms 占用空间：37.5ms
