Check if a Binary Tree contains duplicate subtrees of size 2 or more

Given a Binary Tree, check whether the Binary tree contains a duplicate sub-tree of size 2 or more.
Note : Two same leaf nodes are not considered as subtree size of a leaf node is one.

Input :  Binary Tree 
             /    \ 
           B        C
         /   \       \    
        D     E       B     
                     /  \    
                    D    E
Output : Yes

Asked in : Google Interview

Tree with duplicate Sub-Tree [ highlight by blue color ellipse ]

[ Method 1]
A simple solution is that, we pick every node of tree and try to find is any sub-tree of given tree is present in tree which is identical with that sub-tree. Here we can use below post to find if a subtree is present anywhere else in tree.
Check if a binary tree is subtree of another binary tree

[Method 2 ]( Efficient solution )
An Efficient solution based on tree serialization and hashing. The idea is to serialize subtrees as strings and store the strings in hash table. Once we find a serialized tree (which is not a leaf) already existing in hash-table, we return true.
Below c++ implementation of above idea.

// C++ program to find if there is a duplicate
// sub-tree of size 2 or more.
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 ( == 0)
       return s;

    // Do same for right subtree
    string rStr = dupSubUtil(root->right);
    if ( == 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 "";


    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);

    ("") == 0) ? cout << " Yes ":
                             cout << " No " ;
    return 0;



This article is contributed by Nishant Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

GATE CS Corner    Company Wise Coding Practice

Writing code in comment? Please use, generate link and share the link here.