83. Remove Duplicates from Sorted List Java solutions

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.

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/**
 * 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) {
        if(head == null) return null;
        int tmp = -1000000;
        ListNode res = new ListNode(-1);
        res.next = head;
        ListNode pre = res;
        head = res;
        while(head.next != null){
            if(tmp == head.next.val)
                head.next = head.next.next;
            else{
                tmp = head.next.val;
                head = head.next;
                pre.next = head;
                pre = pre.next;
                
            }
        }
        return res.next;
    }
}