【easy】141. Linked List Cycle

非常简单的题：判断链表有没有环（用快慢指针）

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