leetcode[142]Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

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

/**

 * Definition for singly-linked list.

 * struct ListNode {

 *     int val;

 *     ListNode *next;

 *     ListNode(int x) : val(x), next(NULL) {}

 * };

 */

class Solution {

public:

    ListNode *detectCycle(ListNode *head) {

        if(head==NULL)return NULL;

        ListNode *slow=head;

        ListNode *fast=head;

        bool flag=false;

        while(fast&&fast->next)

        {

            fast=fast->next->next;

            slow=slow->next;

            if(fast==slow)

            {

                flag=true;

                break;

            }

        }

        if(flag)

        {

            fast=head;

            while(fast!=slow)

            {

                fast=fast->next;

                slow=slow->next;

            }

            return fast;

        }

        return NULL;

    }

};