# FAQ: Trees: Conceptual - Binary Search Tree

This community-built FAQ covers the “Binary Search Tree” exercise from the lesson “Trees: Conceptual”.

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## FAQs on the exercise Binary Search Tree

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Something I think the written part is missing an explanation ( I think) is that the numbers need to be greater or lesser than the direct parent above it, but also not greater (if on left side) than any parent upwards and not lesser( if on right side) than any parents upstream.

For example for 23, you can’t have 19 and 50 as children? Is this right? It’s implied in the example but not clearly stated.

Any thoughts from others?

This is a valid binary search tree, so no that’s not right:

``````   23
/  \
19    50
``````

not lesser( if on right side) than any parents upstream.

That’s also not right. If you look at your image 35 is a right child and is lesser than its grandparent, same for 22 and 38

All you need is that smaller values are in the left tree, and larger values are in the right tree, this is what causes the searching to narrow down the candidates by half at each step, because you know which child the sought-for value is in because it’s either smaller or larger than the current node.

Yes, this is what I was stating but the opposite, instead of saying what it should be, what it shouldn’t be. I meant if you were on the right side ( meaning 70 and down) you can’t be lesser than any parent upstream ( can’t be less than 39) . I think we are in agreement.

Thanks, I was guessing through the example, but it’s nice to get validation.

At any point in time you’ll only be looking at a small part of the tree.

And, indeed, it may all be easier to make sense of if you stop trying to think about what happens overall and instead look at what happens at any given moment.

1 Like

BTW, what will happen to a Binary Search Tree if we have two equal values in our “list”?
e.g. [15, 19, 19, 22, 23, 31, 35]
Will the BST look like this?:

``````                                   22
/        \
19         23
/   \       /     \
15   19     31     35
``````

And what if the length of our list is an even number (the number of elements is even)?
How will the BST look like?
Which element to pick as a root node for a list [15, 19, 22, 23, 31, 35]?
22 or 23?

i think this is deep into the maths of the graph theory… Binary trees are artificially created in CS to store data . I think what you guys are talking about is bubbling up values and bubbling down values. I’m sure they will cover it later. You might find William Fiset on Youtube stuff on data structures on trees helpful. He gets a bit more math-y.