在链表中寻找可能的cycle

原题地址

题目

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.

如果一个链表中存在两根指针，一个指针快，另外一个指针慢，如果他们从同样的七点出发，而且这个链表是一个cycle，这两个指针一定会相遇。
同理，现在可以理解为是在不同起点开始出发的两个指针，这两个指针也会相遇的。