Insertion algorithm for binary search tree not working

Question

I am trying to write a function that inserts a node into a binary search tree. I have seen several examples and it seems to me that my algorithm should be working, but it is failing the test for some reason.

def insert(tr, el):
    """ function to insert an element into a binary search tree
    following the rules of binary search trees.
    
    return: an updated tree
    precondition: assumed all elements unique
    """

    if tr == None: 
        return createEyecuBST(el, None)
    elif el < tr.value:
        if tr.left == None:
            tr.left = createEyecuBST(el, tr)
            return tr
        else: return insert(tr.left, el)
    elif el > tr.value:
        if tr.right == None:
            tr.right = createEyecuBST(el, tr)
            return tr
        else: return insert(tr.right, el)
    return None

Are you intending to have an empty function body, or is this a formatting error? — Tritium21
– Tritium21, Commented Nov 15, 2014 at 18:12
@Tritium21 I'm sorry, I dont understand what you mean. Are you saying you can't see what is inside the insert function? — Stalfurion
– Stalfurion, Commented Nov 15, 2014 at 18:15
I am saying, the code as posted has nothing in the function body. its not indented. — Tritium21
– Tritium21, Commented Nov 15, 2014 at 18:16
Ah. I see what you mean that's a formatting error. All the code there should be inside the insert function. I' — Stalfurion
– Stalfurion, Commented Nov 15, 2014 at 18:17
Some nitpicks. Don't check if a value is None with ==, use the is operator. also, don't end your function with return None, python does that for you :) — Tritium21
– Tritium21, Commented Nov 15, 2014 at 18:31

Thibaut · Accepted Answer · 2014-11-15 18:29:32Z

1

There are two lines which appear problematic in your code:

else: return insert(tr.left, el)

and

else: return insert(tr.right, el)

In those cases your function will return a subtree (either the left or the right one) of tr whereas you want your function to return the updated tr tree. I think you should replace these lines by:

else:
    insert(tr.left, el)
    return tr

And similarly for tr.right.

answered Nov 15, 2014 at 18:29

Thibaut

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