LeetCode:Linked List Cycle

题目描述：

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

Follow up:

Can you solve it without using extra space?
思路：初始化一个快指针fast，一个慢指针slow。快指针一次移动两个单位，慢指针一次移动一个单位。如果链表有环，则两指针必然会相遇。否则若fast指针遍历到NULL，则证明链表无环。

代码：

bool hasCycle(ListNode *head) {
ListNode * fast = head;
ListNode * slow = head;
while(fast != NULL && fast->next != NULL)
{
fast = fast->next->next;
slow = slow->next;
if(fast == slow)
break;
}
if(fast == NULL || fast->next == NULL)
return false;
else
return true;
}
};