Untitled

import java.util.*;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode() {}
    TreeNode(int val) { this.val = val; }
    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

public class Solution {
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        List<TreeNode> result = new ArrayList<>();
        Map<String, Integer> subtreeMap = new HashMap<>();
        serializeTree(root, subtreeMap, result);
        return result;
    }

    private String serializeTree(TreeNode node, Map<String, Integer> subtreeMap, List<TreeNode> result) {
        if (node == null) return "#"; // Base case: null nodes are serialized as "#"
        
        // Serialize the current subtree
        String serialized = node.val + "," + 
                            serializeTree(node.left, subtreeMap, result) + "," + 
                            serializeTree(node.right, subtreeMap, result);
        
        // Update the frequency of this serialization
        subtreeMap.put(serialized, subtreeMap.getOrDefault(serialized, 0) + 1);
        
        // If this is the second time we've seen this serialization, add the node to the result
        if (subtreeMap.get(serialized) == 2) {
            result.add(node);
        }
        
        return serialized;
    }
}