Interview Question for Software Engineer at Microsoft:

int SmallestInTree(PNODE root)
{
    if (root == NULL)
    {
        throw;
    }

    int smallest = root->data;
    int smallestInSubtree = smallest;

    if (root->left != NULL)
    {
        smallestInSubtree = SmallestInTree(root->left);
        smallest = (smallestInSubtree < smallest) ?
            smallestInSubtree : smallest;
    }

    if (root->right != NULL)
    {
        smallestInSubtree = SmallestInTree(root->right);
        smallest = (smallestInSubtree < smallest) ?
            smallestInSubtree : smallest;
    }

    return smallest;
}