Replace duplicate values

Replace duplicate values
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Given a linked list that contain some random integers from 1 to n with many duplicates. You need to replace each duplicate element that is present in the linked list with the values n+1, n+2, and so on (starting from left to right in given LL).
For eg. if input linked list is
1 3 1 4 4 2 1 -1
updated linked list should be -
1 3 5 4 6 2 7
n in the list is 4, so I have to replace each duplicate from left to right with 4, 5 and so on. In that way, element 1 (which is at index 2) will be replaced by 5, element 4 (which is present at index 4) by 6 and element 1 (which is present at index 6) by 7.
You don't need to print the elements, just update the elements in LL.
Input format :
Linked list elements (separated by space and terminated by-1)
Output Format :
Updated LL elements (separated by space)
Sample Input 1 :
1 3 1 4 4 2 1 -1
Sample Output 1 :
1 3 5 4 6 2 7

code in java ****************************************************

import java.util.*;
public class solution {

    public static void changeLL(LinkedListNode<Integer> head) {

        Map<Integer,Integer> mymap = new HashMap<>();
        LinkedListNode<Integer> temp = head;
        int maxNum = 0;
        
        while(temp != null){
            mymap.put(temp.data,(mymap.get(temp.data)== null?0:mymap.get(temp.data))+1);
            if(maxNum < temp.data)
                maxNum = temp.data;
            temp = temp.next;
        }

        while(head != null){
            if(mymap.get(head.data)>1)
                mymap.put(head.data,-1);

            else if(mymap.get(head.data)== -1)
                head.data = ++maxNum;

            head = head.next;
        }
    }
}