102. 带环链表

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？

Yes

样例

给出 -21->10->4->5, tail connects to node index 1，返回 true

不要使用额外的空间

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