Leetcode: 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?

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An interesting problem. 

Using two pointers: slow and fast. fast proceeds two nodes/step while slow one node/step. They will meet eventually if there is a loop inside. 

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//codes

class Solution {

public:

       bool hasCycle(ListNode *head) {

              //check input

              if (head == NULL || head->next == NULL)return false;

              //loop

              ListNode *slow = head, *fast = head;

              while (fast != NULL && fast->next != NULL){

                     fast = fast->next->next;

                     slow = slow->next;

                     if (fast == NULL)return false;

                     else if (fast == slow)return true;

              }

              return false;

       }

};