Test5-Replace duplicate values

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

***************************************Solution***************************************

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;
	}
}
}