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?

更简单的环题

/**
 * 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) {
        ListNode *fast = head;
        ListNode *slow = head;
        if(head == NULL || head->next == NULL || head->next->next==NULL)return 0;
        while(1)
        {
            slow = slow->next;
            fast = fast->next;
            if(fast==NULL)return 0;
            fast = fast->next;
            if(fast==NULL)return 0;
            if(fast == slow)return 1;
            
        }
    }
};