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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) {
        //为空
        if(head == NULL || head->next == NULL) {
            return false;
        }
        //快慢指针
        ListNode *fastNode = head->next;
        ListNode *slowNode = head;
        while (fastNode != slowNode) {
            if (fastNode->next == NULL || fastNode->next->next == NULL) {
                return false;
            }
            fastNode = fastNode->next->next;
            slowNode = slowNode->next;
        }
        return true;
    }
};