141. Linked List Cycle

141. Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

第二次遇到这个题 竟然忘记怎么做：

/**
 * 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) {
        //the second time i saw this problem,but...i didn't remember how to solve it
        if (head == nullptr) return false;
        
        auto *slow = head;
        auto *fast = head->next;
        
        while (fast && fast->next && fast != slow) {
            slow = slow->next;
            fast = fast->next->next;
        }
        
        return fast == slow;
    }
};

参考链接：https://leetcode.com/problems/linked-list-cycle/discuss/158085/beats-100(C++)