comefrom: <doocs>0897.Increasing Order Search Tree_1.py
code: def dfs(root):
      if root is None:
          return
      nonlocal prev
      dfs(root.left)
      prev.right = root
      root.left = None
      prev = root
      dfs(root.right)
masked: def sample_funcname ( var_1 ) :
      if var_1 is None :
          return
      nonlocal var_3
      var_0 ( var_1.left )
      var_3.right = var_1
      var_1.left = None
      var_3 = var_1
      var_0 ( var_1.right )
  