Linked List Cycle II

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

/**
 * 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) {
        Map<ListNode,Integer> map = new HashMap<ListNode,Integer>();
        while(head!=null) {
            if(!map.containsKey(head)) {
                map.put(head,1);
                head = head.next;
            }
            else return head;
        }
        return null;
    }
}