code: def test_tree_node_count_unexpanded() -> None:
      tree = _TreeNode(
          param_name="a",
          children={
              0: _TreeNode(
                  param_name="b",
                  children={
                      0.0: _TreeNode(param_name=None, children={}),
                      1.0: _TreeNode(param_name=None, children={}),
                  },
              ),
              1: _TreeNode(
                  param_name="b",
                  children={
                      0.0: _TreeNode(
                          param_name="c",
                          children={
                              0: _TreeNode(
                                  param_name=None, children={}
                              ),
                              1: _TreeNode(),
                          },
                      ),
                      1.0: _TreeNode(),
                  },
              ),
              2: _TreeNode(),
          },
      )
      assert tree.count_unexpanded() == 3
  
file_path: optuna/tests/samplers_tests/test_brute_force.py
imports: import numpy
  import pytest
  import optuna
  from optuna import samplers
  from optuna.samplers._brute_force import _TreeNode
  from optuna.trial import Trial
label_desc: Use image node_id 3 for calling a global function with example usage: test_tree_node_count_unexpanded() without return types