給定一個(gè)鏈表，判斷它是否有環(huán)。

思路：
快指針每次走兩步
慢指針每次走一步
走到最后如果兩指針相遇表示有環(huán)，若快指針走到最后為空，則沒(méi)有環(huán)

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