Linked List Cycle

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?

----------------------------- thinking -------------------------------------------------

the v2's speed is 2 times of v1's speed, then v1 and v2 will meet each other eventually

v*t = d + n1 * L

2*v*t = d + n2 * L

t = (n2 - n1) * L / v;

------------------------------------- codes ---------------------------------------------------

/**

 * 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) {

        if (head == NULL || head->next == NULL || head->next->next == NULL) {

            return false;

        }

        //bug here: the starting v1 and v2 should be 1 and 2 steps ahead, or it will return at if (v1 == v2)

        //v1 = head;

        //v2 = head;

        ListNode *v1 = head->next;

        ListNode *v2 = head->next->next;

        while (v2) {

            if (v1 == v2) {

                return true;

            }

            v1 = v1->next;

            v2 = v2->next;

            v2 = v2 == NULL? NULL:v2->next;

        }

        return false;

    }

};