最大 bst 子树

bool (TreeNode* root) {
        return pd(root, INT_MIN, INT_MAX);
}
bool pd(TreeNode*root, int low, int high) {
        if (root == NULL) return 1;
        if (root->val > low && root->val <high) {
            return pd (root->left, low, root->val) && pd(root->right, root->val, high);
        }
        return 0;
}

int num(TreeNode* root) {
        if (root == NULL) return 0;
        int left = num(root->left);
        int right = num(root->right);
        return left + right + 1;
}

int largestBSTSubtree(TreeNode* root) {
      if (root == NULL) {
          return 0;
      }
      if (bst(root)) {
          return  num(root);
      }
      int left = largestBSTSubtree(root->left);
      int right = largestBSTSubtree(root->right);
      
      return max(left, right);
  }