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 == nullptr || head->next == nullptr) {  // 如果指针为空，则说明没有环
            return false;  
        }  
        
        ListNode *slow = head;  
        ListNode *fast = head->next;  
        
        while (slow != fast) {  
            if (fast == nullptr || fast->next == nullptr) {  // 并且调用空指针会报错
                return false;  
            }  
            slow = slow->next;  
            fast = fast->next->next;  
        }  
        
        return true;  
    }
};