diff --git a/src/my_project/interviews/top_150_questions_round_20/count_complete_tree_nodes.py b/src/my_project/interviews/top_150_questions_round_20/count_complete_tree_nodes.py
new file mode 100644
index 00000000..515b9bda
--- /dev/null
+++ b/src/my_project/interviews/top_150_questions_round_20/count_complete_tree_nodes.py
@@ -0,0 +1,17 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def countNodes(self, root: Optional[TreeNode]) -> int:
+
+        if not root:
+            return 0
+        else: 
+            return self.countNodes(root.left) + self.countNodes(root.right) + 1
\ No newline at end of file
diff --git a/tests/test_150_questions_round_20/test_count_complete_tree_nodes_round_20.py b/tests/test_150_questions_round_20/test_count_complete_tree_nodes_round_20.py
new file mode 100644
index 00000000..f7d3c15b
--- /dev/null
+++ b/tests/test_150_questions_round_20/test_count_complete_tree_nodes_round_20.py
@@ -0,0 +1,18 @@
+import unittest
+from src.my_project.interviews.top_150_questions_round_20\
+.count_complete_tree_nodes import TreeNode, Solution
+
+class CountNodesTestCase(unittest.TestCase):
+
+    def test_count_none(self):
+        solution = Solution()
+        tree = None 
+        output = solution.countNodes(root=tree)
+        self.assertEqual(0, output)
+
+
+    def test_count_non_empty_tree(self):
+        solution = Solution()
+        tree = TreeNode(1, TreeNode(2), TreeNode(3))
+        output = solution.countNodes(root=tree)
+        self.assertEqual(3, output)
\ No newline at end of file