Duplicate subtree in Binary Tree

//User function Template for Java

/* A Binary Tree node

class Node
{
    int data;
    Node left, right;

    Node(int item)
    {
        data = item;
        left = right = null;
    }
}
 */

class Solution {
    
    
    public void getNodeList(Node node, ArrayList<Node> nodeList) 
    {
        if(node == null) return;
        
        nodeList.add(node);
        getNodeList(node.left, nodeList);
        getNodeList(node.right, nodeList);
        
    }
    
    public void helper(Node node, StringBuilder pattern) 
    {
        if(node == null) return;
        
        pattern.append(node.data);
        helper(node.left, pattern);
        helper(node.right, pattern);
    }
    
    
    
    
    
    
    
    
    
    
    int dupSub(Node root) 
    {
        // code here 
        
        ArrayList<Node> nodes = new ArrayList<>();
        HashSet<String> hs = new HashSet<>();
        
        getNodeList(root, nodes);
        
        for(Node n : nodes)
        {
            StringBuilder pattern = new StringBuilder();
            
            helper(n, pattern);
            
            if(hs.contains(pattern.toString()) && pattern.length() >= 3) 
            {
                return 1;
            }
            
            hs.add(pattern.toString());
            
            
        }
        return 0;
    }
}