Linked List Cycle
Given a linked list, determine if it has a cycle in it.
Follow up: Can you solve it without using extra space?
如何判断一个单链表中有环？
Linked List Cycle II
Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
Follow up: Can you solve it without using extra space?
如何找到环的第一个节点？