int tree :: elem_with_one_child(tree_node *root)
{
    int count=0;
    if(!root)
        return 0;
    if(!root->left && !root->right)
        return 0;
    if((root->left == 0 && root->right) || (root->left && root->right == 0))
    {
        if(root->left)
            count+=elem_with_one_child(root->left);
        if(root->right)
            count+=elem_with_one_child(root->right);
        count++;
    }
    else
    {
        count+=elem_with_one_child(root->right);
        count+=elem_with_one_child(root->left);
    }
    return count;
}