comefrom: <doocs>0098.Validate Binary Search Tree_1.py
code: def dfs(root):
      nonlocal prev
      if root is None:
          return True
      if not dfs(root.left):
          return False
      if prev >= root.val:
          return False
      prev = root.val
      if not dfs(root.right):
          return False
      return True
masked: def sample_funcname ( var_1 ) :
      nonlocal var_2
      if var_1 is None :
          return True
      if not var_0 ( var_1.left ) :
          return False
      if var_2 >= var_1.val :
          return False
      var_2 = var_1.val
      if not var_0 ( var_1.right ) :
          return False
      return True
  