Linked List Cycle II

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

null

.

Note: Do not modify the linked list.

Follow up:

Can you solve it without using extra space?

Subscribe to see which companies asked this question
/**
* 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 head;
ListNode* slow=head;
ListNode* fast=head;
while(fast!=NULL && fast->next!=NULL)
{
slow=slow->next;
fast=fast->next->next;
if(slow==fast)
break;
}
if(fast==NULL || fast->next==NULL) return NULL;
slow=head;
while(slow!=fast)
{
slow=slow->next;
fast=fast->next;
}
return fast;
}
};