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?

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;
    }
};

LeetCode:Linked List Cycle