LeetCode OJ：Linked List Cycle（链表循环）

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

判断一个链表是否存在环，维护快慢指针就可以，如果有环那么快指针一定会追上慢指针，代码如下：

 class Solution {
 public:
     bool hasCycle(ListNode *head) {
         ListNode * slow, * fast;
         slow = fast = head;
         while(slow && fast){
             slow = slow->next;
             fast = fast->next;
             if(fast) fast = fast->next;
             if(fast && slow && slow == fast)
                 return true;
         }
         return false;
     }
 };