comefrom: <doocs>0783.Minimum Distance Between BST Nodes_1.py
code: def dfs(root):
      if root is None:
          return
      dfs(root.left)
      nonlocal ans, prev
      ans = min(ans, abs(prev - root.val))
      prev = root.val
      dfs(root.right)
masked: def sample_funcname ( var_1 ) :
      if var_1 is None :
          return
      var_0 ( var_1.left )
      nonlocal var_2 , var_3
      var_2 = min ( var_2 , abs ( var_3 - var_1.val ) )
      var_3 = var_1.val
      var_0 ( var_1.right )
  