题解 | #判断链表中是否有环#

/**
 * 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 || head ->next == nullptr) return false;
        ListNode * fast = head;
        ListNode * slow = head;
        while(fast){   
            slow = slow->next;
            fast = fast->next;
            if(!fast) return false;
            fast = fast->next;//在使用指针一次向后指两次时，需要注意得先判断一下好、fast->next 是否为空，不为空才能进一步向后指
            if(fast ==  slow) return true;
        }
        return false;
        
    }
};