141 环形链表

https://leetcode.cn/leetbook/read/top-interview-questions-easy/xnwzei/

https://leetcode.cn/problems/linked-list-cycle/description/

cost: 2m21s

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

• 时间复杂度：O(n)
• 空间复杂度：O(1)