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.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {	
    	if(head==null||head.next==null) return null;
    	ListNode p=head;
    	ListNode q=head;
    	while(q!=null){
    		p=p.next;
    		if(q.next!=null) q=q.next.next;
    		else return null;
    		if(p==q) break;
    	}
    	if(q==null) return null;
    	
    	p=head;
    	while(p!=q){
    		p=p.next;
    		q=q.next;
    	}
    	 return p;
    }
}