Linked List Cycle

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

Example

Given -21->10->4->5, tail connects to node index 1, return true

Challenge

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

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/** * Definition of ListNode * class ListNode { * public: * int val; * ListNode *next; * ListNode(int val) { * this->val = val; * this->next = NULL; * } * } */ class Solution { public: /** * @param head: The first node of linked list. * @return: True if it has a cycle, or false */ bool hasCycle(ListNode *head) { // write your code here if (!head || !head->next) return false; ListNode *slow=head; ListNode *fast=head->next; while(fast && fast->next){ if (slow==fast) return true; slow=slow->next; fast=fast->next->next; } return false; } };