判断是否为bst

class  {
public:
    bool isValidBST(TreeNode* root) {
        if (root == nullptr) {
            return true;
        }
        stack<TreeNode*> stackNode;
        TreeNode* prev = nullptr;
        while (root != nullptr || !stackNode.empty()) {
            while (root != nullptr) {
                stackNode.push(root);
                root = root->left;
            }
            root = stackNode.top();
            stackNode.pop();
            if (prev != nullptr && root->val <= prev->val) {
                return false;
            }
            prev = root;
            root = root->right;
        }
        return true;
    }
};