程序代码：

bool isBST(struct tree *root)
{
    if (root == NULL)
        return true;
    if (isBST(root->left) && isBST(root->right))
        return (root->left == NULL ? true : (root->left->v < root->v))
                && (root->right == NULL ? true : (root->v < root->right->v));
}

程序代码：

bool isBST(struct tree *root)
{
    static int preV = INT_MIN;
    if (root == NULL)
        return true;
    isBST(root->left);
    if (root->v > preV)
        preV = root->v;
    else
        return false;
    isBST(root->right);
}

程序代码：

bool isBST(struct tree *root)
{
    static int preV = INT_MIN;
    if (root == NULL)
        return true;
    if (!isBST(root->left))
        return false;
    if (root->v > preV)
        preV = root->v;
    else
        return false;
    isBST(root->right);
}