LeetCode-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.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode dummy=new ListNode(0);
        dummy.next=head;
        ListNode dp=dummy.next;
        ListNode previous=null;
        while(dp!=null){
            if(previous!=null){
                if(dp.val==previous.val){
                    previous.next=dp.next;
                }
                else{
                    previous=dp;
                }
            }
            else{
                previous=dp;
            }
            dp=dp.next;
        }
        return dummy.next;
    }
}