comefrom: <doocs>0449.Serialize and Deserialize BST_1.py
code: def dfs(root):
      if root is None:
          return
      nonlocal t
      t.append(str(root.val))
      t.append(',')
      dfs(root.left)
      dfs(root.right)
masked: def sample_funcname ( var_1 ) :
      if var_1 is None :
          return
      nonlocal var_3
      var_3.append ( str ( var_1.val ) )
      var_3.append ( ',' )
      var_0 ( var_1.left )
      var_0 ( var_1.right )
  