Add depth function

lib/mb_binary_tree.ex

@@ -97,4 +97,28 @@ defmodule MbBinaryTree do
       [12, nil, [4, nil, nil]]
   """
   def update_right([node, left, _right], new_right), do: [node, left, new_right]
+
+  @doc """
+  Calculate the depth of a binary tree.
+
+  ## Examples
+
+      iex> MbBinaryTree.depth(nil)
+      0
+      iex> MbBinaryTree.depth([1, nil, nil])
+      1
+      iex> MbBinaryTree.depth([1, [2, nil, nil], [3, nil, nil]])
+      2
+      iex> MbBinaryTree.depth([1, [2, nil, nil], [3, [4, nil, nil], [5, nil, nil]]])
+      3
+      iex> MbBinaryTree.depth([12, [4, MbBinaryTree.new(2), MbBinaryTree.new(5)], [26, MbBinaryTree.new(14), [28, MbBinaryTree.new(27), MbBinaryTree.new(30)]]])
+      4
+  """
+  def depth(tree) do
+    case tree do
+      nil -> 0
+      [_, nil, nil] -> 1
+      [_, left, right] -> 1 + max(depth(left), depth(right))
+    end
+  end
 end