class NotNullStack: private vector<TreeNode *> {
    public:
        static NotNullStack make_stack(TreeNode *node) 
        {
            if(node==nullptr)
                return NotNullStack {};
            else
                return NotNullStack {node};
        }
        void push(TreeNode *node) 
        {
            if(node!=nullptr)
                vector::push_back(node);
        }
        TreeNode *pop()
        {
            if(empty())
                return nullptr;
            TreeNode *result = back();
            pop_back();
            return result;
        }
        using vector::empty;
    private:
        NotNullStack() {}
        NotNullStack(std::initializer_list<TreeNode *> init) : vector {init} {}
};

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        NotNullStack stack = NotNullStack::make_stack(root);
        vector<int> result;
        for(TreeNode* node = stack.pop(); node!=nullptr; node = stack.pop())
        {
            result.push_back(node->val);
            stack.push(node->right);
            stack.push(node->left);
        }
        return result;
    }
};