eliminate the duplicates in the sorted list (2)

Question:

Given a sorted list, and remove the duplicates in the list. For example, 1 -> 2 -> 2 -> 3, after removing the duplicates, we have  1 -> 2  -> 3

public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode current = head.next;
        ListNode previous = head;
        previous.next = null;
        while (current != null) {
            if (current.val != previous.val) {
                previous.next = current;
                previous = current;
                current = current.next;
                previous.next = null;
            } else {
                current = current.next;
            }
        }
        return head;
    }
}

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