LeetCode:Linked List Cycle

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

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

/**
 * 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) {
            if(head==NULL)
                return false;
            ListNode *p=head,*q;
            int n=0,m;
            while(p!=NULL)
            {
                q=head;
                m=0;
                while(m<=n)
                {
                    if(q==p->next)
                        return true;
                    else
                    {
                        q=q->next;
                        m++;
                    }
                }
                p=p->next;
                n++;
            }
            return false;
        }
};

后记：在做Linked List Cycle II 的时候，发现上述解法效率太低，所以换了种思路：

（1）定义slow和fast指针，都指向head结点，然后slow指针每次向后走1个结点，fast指针每次向后走2个结点。

（2）如果fast指针为空，则说明不存在环；如果fast指针和slow指针相等，则说明存在环。

/**
 * 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) {
            if(head==NULL||head->next==NULL)
                return false;
            ListNode *slow=head,*fast=head;
            while(fast!=NULL)
            {
                slow=slow->next;
                fast=fast->next;
                if(fast!=NULL)
                    fast=fast->next;
                if(slow==fast)
                    return true;
            }
            return false;
        }
};