From 629848e3721d9354d25fad6cb4729e6afdbbf799 Mon Sep 17 00:00:00 2001
From: Sherman Hui <11592023+shermanhui@users.noreply.github.com>
Date: Fri, 22 Oct 2021 07:07:05 -0700
Subject: [PATCH] [mypy] Fix type annotations in `data_structures/binary_tree`
 (#5518)

* fix: fix mypy errors

Update binary_search_tree `arr` argument to be typed as a list
within `find_kth_smallest` function

Update return type of `merge_two_binary_trees` as both inputs can
be None which means that a None type value can be returned from
this function

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
---
 DIRECTORY.md                                          | 1 +
 data_structures/binary_tree/binary_search_tree.py     | 2 +-
 data_structures/binary_tree/merge_two_binary_trees.py | 4 +++-
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/DIRECTORY.md b/DIRECTORY.md
index 10149eac5..950d8e2c0 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -272,6 +272,7 @@
 
 ## Electronics
   * [Carrier Concentration](https://github.com/TheAlgorithms/Python/blob/master/electronics/carrier_concentration.py)
+  * [Coulombs Law](https://github.com/TheAlgorithms/Python/blob/master/electronics/coulombs_law.py)
   * [Electric Power](https://github.com/TheAlgorithms/Python/blob/master/electronics/electric_power.py)
   * [Ohms Law](https://github.com/TheAlgorithms/Python/blob/master/electronics/ohms_law.py)
 
diff --git a/data_structures/binary_tree/binary_search_tree.py b/data_structures/binary_tree/binary_search_tree.py
index a1ed1d0ac..ce490fd98 100644
--- a/data_structures/binary_tree/binary_search_tree.py
+++ b/data_structures/binary_tree/binary_search_tree.py
@@ -151,7 +151,7 @@ class BinarySearchTree:
 
     def find_kth_smallest(self, k: int, node: Node) -> int:
         """Return the kth smallest element in a binary search tree"""
-        arr = []
+        arr: list = []
         self.inorder(arr, node)  # append all values to list using inorder traversal
         return arr[k - 1]
 
diff --git a/data_structures/binary_tree/merge_two_binary_trees.py b/data_structures/binary_tree/merge_two_binary_trees.py
index d169e0e75..748726894 100644
--- a/data_structures/binary_tree/merge_two_binary_trees.py
+++ b/data_structures/binary_tree/merge_two_binary_trees.py
@@ -7,6 +7,8 @@ will be used as the node of new tree.
 """
 from __future__ import annotations
 
+from typing import Optional
+
 
 class Node:
     """
@@ -19,7 +21,7 @@ class Node:
         self.right: Node | None = None
 
 
-def merge_two_binary_trees(tree1: Node | None, tree2: Node | None) -> Node:
+def merge_two_binary_trees(tree1: Node | None, tree2: Node | None) -> Optional[Node]:
     """
     Returns root node of the merged tree.