[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;
    }
};