[LeetCode]Link List Cycle

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

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

思考：快慢指针，快指针一次走两步，慢指针一次一步。若快指针跟慢指针指向同一个结点，则有环。若快指针到达链表末尾即指向NULL，说明没有环。

/**

 * Definition for singly-linked list.

 * struct ListNode {

 *     int val;

 *     ListNode *next;

 *     ListNode(int x) : val(x), next(NULL) {}

 * };

 */

class Solution {

public:

    bool hasCycle(ListNode *head) {

        // IMPORTANT: Please reset any member data you declared, as

        // the same Solution instance will be reused for each test case.

		if(!head||!head->next) return false;

		ListNode *p=head;

		ListNode *q=p->next;

		while(q&&q->next)

		{

			if(p==q) return true;

			else

			{

				q=q->next->next;

				p=p->next;

			}

		}

        return false;

    }

};