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) {
            return false;
        }

        auto slow = head;
        auto fast = head->next->next;
        while (slow != fast) {
            slow = slow->next;
            if (fast == nullptr) {
                return false;
            }
            fast = fast->next;
            if (fast != nullptr) {
                fast = fast->next;
            } else {
                return false;
            }
        }
        return true;        
    }
};