Duplicates in binary tree

// C++ program to find if there is a duplicate 
// sub-tree of size 2 or more. 
#include<bits/stdc++.h> 
using namespace std; 
  
// Separator node 
const char MARKER = '$'; 
  
// Structure for a binary tree node 
struct Node 
{ 
    char key; 
    Node *left, *right; 
}; 
  
// A utility function to create a new node 
Node* newNode(char key) 
{ 
    Node* node = new Node; 
    node->key = key; 
    node->left = node->right = NULL; 
    return node; 
} 
  
unordered_set<string> subtrees; 
  
// This function returns empty string if tree 
// contains a duplicate subtree of size 2 or more. 
string dupSubUtil(Node *root) 
{ 
    string s = ""; 
  
    // If current node is NULL, return marker 
    if (root == NULL) 
        return s + MARKER; 
  
    // If left subtree has a duplicate subtree. 
    string lStr = dupSubUtil(root->left); 
    if (lStr.compare(s) == 0) 
       return s; 
  
    // Do same for right subtree 
    string rStr = dupSubUtil(root->right); 
    if (rStr.compare(s) == 0) 
       return s; 
  
    // Serialize current subtree 
    s = s + root->key + lStr + rStr; 
  
    // If current subtree already exists in hash 
    // table. [Note that size of a serialized tree 
    // with single node is 3 as it has two marker 
    // nodes. 
    if (s.length() > 3 && 
        subtrees.find(s) != subtrees.end()) 
       return ""; 
  
    subtrees.insert(s); 
  
    return s; 
} 
  
// Driver program to test above functions 
int main() 
{ 
    Node *root = newNode('A'); 
    root->left = newNode('B'); 
    root->right = newNode('C'); 
    root->left->left = newNode('D'); 
    root->left->right = newNode('E'); 
    root->right->right = newNode('B'); 
    root->right->right->right = newNode('E'); 
    root->right->right->left= newNode('D'); 
  
    string str = dupSubUtil(root); 
  
    (str.compare("") == 0) ? cout << " Yes ": 
                             cout << " No " ; 
    return 0; 
}