Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
分析：
判断一个链表是否为环形链表，且不要使用额外空间。判断环形链表通常利用快慢指针来做，快指针一次走两步，慢指针一次走一步，若存在环，则快指针定会与慢指针相遇，
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
//利用快慢指针
public:
bool hasCycle(ListNode *head) {
ListNode *slow = head;
ListNode *fast = head;
while(fast && fast->next)
{
slow = slow->next;
fast = fast->next->next;
if(slow == fast)
return true;
}
return false;
}
};