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 || head->next==NULL) return NULL;
ListNode *fast = head, *slow = head;
while(fast && fast->next){
fast = fast->next->next;
slow = slow->next;
if(fast == slow) break;
}
if(fast!=slow) return NULL;
slow = head;
while(slow!=fast){ slow = slow->next; fast = fast->next; }
return slow;
}
};