feat: adds max_depth_binary_tree exercise

Author: flamiedev

max_depth_binary_tree.py

@@ -0,0 +1,19 @@
+from typing import Optional
+
+
+# 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 maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        else:
+            left_depth = self.maxDepth(root.left)
+            right_depth = self.maxDepth(root.right)
+            return max(left_depth, right_depth) + 1