【算法】快慢指针（主要是链表）------leetcode和offer应用【待总结】

leetcode：

https://blog.youkuaiyun.com/chenxiyuehh/article/details/90241722

offer：

https://blog.youkuaiyun.com/qq_30193419/article/details/93596329

https://blog.youkuaiyun.com/qq_21815981/article/details/79833976（更多例题）

bool HasCircle(Node *head)
{
  if(head == NULL)
    return false;
  Node *slow = head, *fast = head;
  while(fast != NULL && fast->next!=NULL)
  {
     slow = slow->next; //慢指针每次前进一步
     fast = fast->next->next;//快指针每次前进两步
     if(slow == fast) //相遇，存在环
         return true;
  }
  return false;
}