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. * Class ListNode {* int val; * ListNode Next; * ListNode (int x) {* val = x; * next = NULL; *     } * } */ Public classSolution { PublicListNode detectcycle (ListNode head) {ListNode fast=Head; ListNode Slow=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; } Fast=Head;  while(fast!=slow) {Fast=Fast.next; Slow=Slow.next; }        returnFast; }                                /*ListNode Fast=head;        ListNode Slow=head; if (head==null| |        Head.next==null) return null;            while (fast!=null) {slow=slow.next;            if (fast.next!=null) {fast=fast.next.next;            }else return null;                    if (slow==fast) break;            } if (slow==fast) {fast=head;                while (Fast!=slow) {fast=fast.next;            Slow=slow.next;                    } return fast;                    } return null; }*/}

Linked List Cycle II