/**
 * Definition for a binary tree node.
 * public 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;
 *     }
 * }
 */
class Solution {
    
    
    Map<String,Integer> count;
    List<TreeNode> ans;
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
    
    
        count = new HahMap();
        ans = new ArrayList();
        collect(root);
        return ans;
    }
    public String collect(TreeNode node){
    
    
        if(node == null) return "#";
        String serial = node.val + "," + collect(node.left)+","+collect(node.right);
        count.put(serial,count.getOrDefault(serial,0)+1);
        if(count.get(serial) == 2){
    
    
            ans.add(node);
        }
        return serial;
    }
}