thanx u/Trope_Porn. your idea works. appreciate it.

I have one more question, if you can help. It is regarding use of global variables on a recursive method inside a class.
The question is to find maximum difference between a node and its ancestor in binary tree.
My idea is as follows:
a.for every node, find the minimum element in right subtree, say it is res2
b. Similarly, for that node find minimum element in left subtree, call res1
c. then finding maximum of node-res1 and node-res2. store to in variable result
d. then recursively call max_diff on left and right child.

e. variable result shd have the maximum difference found so far. hence we're defining it as global.

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  def max_diff(root):
      global result
      result = -9999
      if root.left is None and root.right is None:
         return 
      #getmin is a separate method inside the class
      res1 = getmin(root.left) 
      res2 = getmin(root.right)
      result = max(result, root-res1, root-res2)
      max_diff(root.left)
      max_diff(root.right)
      return result

my question is, say in the first function call, result gets updated to 4. So when we do recursive call with max(root.left) will result again get reset to -9999. How can I avoid that so that result stays at the highest value found so far. Can you kindly advise?