Find Duplicate Subtrees

Two trees are duplicate if they have the same structure with same node values.

import java.util.HashMap;
public class Duplicate_subtress {

    /* A binary tree node has data, pointer to
    left child and a pointer to right child */
    static HashMap<String, Integer> m;
    static class Node {
        int data;
        Node left;
        Node right;
        Node(int data){
            this.data = data;
            left = null;
            right = null;
        }
    }
    static String inorder(Node node)
    {
        if (node == null)
            return "";

        String str = "(";
        str += inorder(node.left);
        str += Integer.toString(node.data);
        str += inorder(node.right);
        str += ")";

        // Subtree already present (Note that we use
        // HashMap instead of HashSet
        // because we want to print multiple duplicates
        // only once, consider example of 4 in above
        // subtree, it should be printed only once.       
        if (m.get(str) != null && m.get(str)==1 )
            System.out.print( node.data + " ");

        if (m.containsKey(str))
            m.put(str, m.get(str) + 1);
        else
            m.put(str, 1);


        return str;
    }

    // Wrapper over inorder()
    static void printAllDups(Node root)
    {
        m = new HashMap<>();
        inorder(root);
    } 
    // Driver code
    public static void main(String args[])
    {
        Node root = null;
        root = new Node(1);
        root.left = new Node(2);
        root.right = new Node(3);
        root.left.left = new Node(4);
        root.right.left = new Node(2);
        root.right.left.left = new Node(4);
        root.right.right = new Node(4);
        printAllDups(root);
    }
}