Untitled

    // Iterative Preorder Traversal
    vector<int> preorderTraversal(TreeNode* root) {
        TreeNode *curr = root;
        stack<TreeNode *> st;
        vector<int> res;

        while (!st.empty() || curr != NULL) {
            if (curr != NULL) {
                res.push_back(curr -> val);
                st.push(curr);
                curr = curr -> left;
            } else {
                curr = st.top();
                st.pop();
                curr = curr -> right;
            }
        }

        return res;
    }