Comments on: Programming Interview Questions 26: Trim Binary Search Tree

By: Kowshik

Kowshik — Thu, 19 Jan 2012 10:05:22 +0000

Sorry, please ignore. It works fine!
Great question!

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By: Kowshik

Kowshik — Thu, 19 Jan 2012 10:01:49 +0000

It looks to me like there are null checks missing (or None checks in Python?).
Please see my comment inline below:

def trimBST(tree, minVal, maxVal):
if not tree:
return
tree.left=trimBST(tree.left, minVal, maxVal)
tree.right=trimBST(tree.right, minVal, maxVal)
##########
# Shouldn’t there be a check here on tree.left == None and tree.right == None
# before the comparisons below? This happens when leaf nodes are checked and
# the first line returns None viz:
#
# if not tree:
# return
##########
if minVal<=tree.val<=maxVal:
return tree
if tree.valmaxVal:
return tree.left

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By: Arden

Arden — Tue, 17 Jan 2012 23:17:03 +0000

But note that we still have to destroy all that subtree, otherwise a memory leak will occur. So we can’t simply cut the link between the node and its child. In python code I don’t explicitly free the nodes, I just remove references to them and then the garbage collector performs the deletion itself. But I will update the article mentioning this case to avoid further confusion. Thanks for the comment, this is important to note.

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By: Sachin

Sachin — Tue, 17 Jan 2012 21:26:43 +0000

If we do the condition checking before the recursive calls, we can avoid one unnecessary recursive call for the cases where tree.val > max and tree.val < min. When tree.val > max, we need to trim only the left subtree and vice versa.

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