判断给定的链表中是否有环

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