void printByLevel(TreeNode* root) {
if (!root) return;

    queue<TreeNode*> q;
    q.push(root);

    while (!q.empty()) {
        for (int i = 0; i < q.size(); ++i) {
            TreeNode* cur = q.front();
            q.pop();
            cout << cur->val << " ";

            if (cur->left) q.push(cur->left);
            if (cur->right) q.push(cur->right);
        }
        cout << endl;
    }
}