void topView(Node* root)
    {
        if (!root)  return;

        map<Node*, int> hd;
        map<int, int> m;
        deque<Node*> q;
        q.push_back(root);

        hd[root]=0;
        m[hd[root]]=root->data;
        while(!q.empty())
        {
            if (root->left)
            {
                hd[root->left] = hd[root]-1;
                if (!m[hd[root->left]]) m[hd[root->left]] = root->left->data;
                q.push_back(root->left);
            }
            if (root->right)
            {
                hd[root->right] = hd[root]+1;
                if (!m[hd[root->right]]) m[hd[root->right]] = root->right->data;
                q.push_back(root->right);            
            }
            q.pop_front();
            if (q.size())   root = q.front();
        }
        map<int, int>::iterator it = m.begin();

        while(it!=m.end())
        {
            cout << it->second << " ";
            it++;
        }
    }