title: the answer variable is not being updated in max depth of binary tree question
body: <p>I was trying to solve this max depth of the binary tree question using top-down approach. However, for some reason my answer variable is not being updated. My answer variable which is supposed to print the max depth returns 0. I am not able to understand why this is happening?</p>
  <pre><code>def maxDepthHelper(root, depth, answer) -&gt; int:
      if root is None:
          return 
      if root.left is None and root.right is None:
          answer = max(answer, depth)
          return
          
      maxDepthHelper(root.left, depth + 1, answer)
      print(answer)
      maxDepthHelper(root.right, depth + 1, answer)
  
  
  class Solution:
      def maxDepth(self, root: Optional[TreeNode]) -&gt; int:
          answer = 0
          maxDepthHelper(root, 1, answer)
          return answer
      
  </code></pre>