/**
 * 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 {
    Set<TreeNode> ans = new HashSet<>();
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        helper(root.left, root.right);
        return new ArrayList(ans);
    }

    public boolean helper(TreeNode node1, TreeNode node2) {
        if(node1 == null && node2 == null) {
            return true;
        } else if((node1 == null && node2 != null ) || (node1 != null && node2 == null)) {
            return false;
        }
        boolean answer = false;
        if(node1 != null && node2 != null) {
            if(node1.val == node2.val) {
                answer = helper(node1.left, node2.left) && helper(node1.right, node2.right);
                if(answer) {
                    ans.add(node1);
                    return true;
                }
            }

        }
        if(!answer) {
            answer = helper(node1, node2.left) || helper(node1, node2.right);
        }
        if(!answer) {
            answer = helper(node1.left, node2) || helper(node1.right, node2);
        }
        return answer;
    }
}