Boundry Traversal in Binary tree

How to perform boundry traversal in binary tree using only single traversal of the tree??

i am doing it in this way.. Not a single traversal

void printBoundary (struct node* root) { if (root) { printf("%d ",root->data);

    // Print the left boundary in top-down manner.
    if( root->left)
        printBoundaryLeft(root->left);
    else
        printBoundaryLeft(root->right);

    // Print all leaf nodes
    printLeaves(root->left);
    printLeaves(root->right);

    // Print the right boundary in bottom-up manner
    if( root->right)
       printBoundaryRight(root->right);
    else
       printBoundaryRight(root->left);
}

}