struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x): val(x), left(nullptr), right(nullptr) {}
};

void dfs(TreeNode* root) {
    if (!root) return;
    stack<TreeNode*> st;
    st.push(root);
    while (!st.empty()) {
        TreeNode* node = st.top(); st.pop();
        cout << node->val << " ";
        if (node->right) st.push(node->right);
        ________________
    }
}