void top_view(node * root)
{
    node * tmp = NULL;
    if(root->left){
        tmp = root->left->right;
        root->left->right = NULL;
        top_view(root->left);
        root->left->right = tmp;
    }
    cout<<root->data<<" ";
    if(root->right){
        tmp = root->right->left;
        root->right->left = NULL;
        top_view(root->right);
        root->right->left = tmp;
    }
    return;
}