题目

题解

1. 哈希表

class Solution {
public:
    bool hasCycle(ListNode *head) {
        unordered_set<ListNode*> seen;
        while (head != nullptr) {
            if (seen.count(head)) {
                return true;
            }
            seen.insert(head);
            head = head->next;
        }
        return false;
    }
};

2.Floyd 判圈算法

class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (head == nullptr || head->next == nullptr) {
            return false;
        }
        ListNode * fast = head->next;
        ListNode * slow = head;
        while (slow != fast) {
            if (fast == nullptr || fast->next == nullptr)return false;
            slow = slow->next;
            fast = fast->next->next;
        }
        return true;
    }
};