Leetcode 中Linked List Cycle 一类问题

/**
 * 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* slow = head;
        ListNode* fast = head;
        while (fast -> next != NULL && fast -> next -> next != NULL){    // 注意判断条件，第一次写的时候没有注意到要判断fast -> next是否为空
            slow = slow -> next;
            fast = fast -> next -> next;
            if (fast == slow){
                return true;
            }
        }
        return false;
    }
};