Leetcode | | 141. Linked List Cycle

Problem:

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

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

Hide TagsLinked List Pointers

Thinking:

(1) If additional space can be opened, using Unordered_set to store traversed nodes, there is a ring structure when there is repetition.

(2) If the extra space is not applicable and the space complexity is O (1), use fast and slow double pointers. The slow pointer goes one step at a time, and the quick pointer goes two steps at a time.

If there is a ring structure, two pointers will always meet.

(3) Termination conditions should also be noted:

Fast!=null && Fast->next!=null

Prevent Fast->null->next from appearing

Code

/** * Definition for singly-linked list. * struct ListNode {*     int val; *     ListNode *next; *     listnode (int x): Val (x), Next (NULL) {} *}; */class Soluti On {public:    bool Hascycle (ListNode *head) {          ListNode *fast,*slow;         if (head==null) return false;         Slow=head;         fast=head->next;         while (Fast!=null && fast->next!=null)         {             if (slow==fast) return true;             slow=slow->next;             fast=fast->next->next;         }         return false;    }};

Leetcode | | 141, Linked List Cycle