comefrom: <doocs>0426.Convert Binary Search Tree to Sorted Doubly Linked List_1.py
code: def dfs(root):
      if root is None:
          return
      nonlocal prev, head
      dfs(root.left)
      if prev:
          prev.right = root
          root.left = prev
      else:
          head = root
      prev = root
      dfs(root.right)
masked: def sample_funcname ( var_1 ) :
      if var_1 is None :
          return
      nonlocal var_3 , var_4
      var_0 ( var_1.left )
      if var_3 :
          var_3.right = var_1
          var_1.left = var_3
      else :
          var_4 = var_1
      var_3 = var_1
      var_0 ( var_1.right )
  