Remove Duplicates from Sorted List

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->2->3.

Solution:
Use Hash Map to check duplicates.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (head == null) return null;
        HashMap hash = new HashMap();
        hash.put(head.val,true);
        ListNode prev = head;
        ListNode node = head.next;
        while(node != null){
            if(!hash.containsKey(node.val)){
                hash.put(node.val,true);
                prev = node;
            }
            else{
                prev.next = node.next;
            }
            node = node.next;
        }
        return head;
    }
}