链表-带环链表-中等

描述
给定一个链表，判断它是否有环。
您在真实的面试中是否遇到过这个题？  是
样例
给出 -21->10->4->5, tail connects to node index 1，返回 true
挑战

不要使用额外的空间

题目链接

程序

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */


class Solution {
public:
    /*
     * @param head: The first node of linked list.
     * @return: True if it has a cycle, or false
     */
    bool hasCycle(ListNode * head) {
        // write your code here
        if(head == NULL || head->next == NULL)
            return false;
        ListNode *slow = head, *fast = head->next;
        while(fast!= NULL && fast->next != NULL){
            if(fast == slow)
                return true;
            fast = fast->next->next;
            slow = slow->next;
        }
        return false;
    }
};