Trie (Prefix Tree) in Python


I don't know if this is the place to ask about algorithms. But let's see if I get any answers ... :)

If anything is unclear I'm very happy to clarify things.

I just implemented a Trie in python. However, one bit seemed to be more complicated than it ought to (as someone who loves simplicity). Perhaps someone has had a similar problem?

My aim was to minimize the number of nodes by storing the largest common prefix of a sub-trie in its root. For example, if we had the words stackoverflow, stackbase and stackbased, then the tree would look something like this:

[o]verflow ______/ \_______ [b]ase
                                  \___ [d]

Note that one can still think of the edges having one character (the first one of the child node).

Find-query is simple to implement. Insertion is not hard, but somewhat more complex than I want.. :(

My idea was to insert the keys one after the other (starting from an empty trie), by first searching for the to-be-inserted key k (Find(k)), and then rearranging/splitting the nodes locally at the place where the find-procedure stops. There turn out to be 4 cases: (Let k be the key we want to insert, and k' be the key of the node, where the search ended)

  1. k is identical to k'
  2. k is a "proper" prefix of k'
  3. k' is a "proper" prefix of k
  4. k and k' share some common prefix, but none of the cases (1), (2) or (3) occur.

It seems that each of the cases are unique and thus imply different modifications of the Trie. BUT: is it really that complex? Am I missing something? Is there a better approach?

Thanks :)

6/7/2009 1:14:07 AM

Accepted Answer

At a glance, it sounds like you've implemented a Patricia Trie. This approach also is called path compression in some of the literature. There should be copies of that paper that aren't behind the ACM paywall, which will include an insertion algorithm.

There's also another compression method you may want to look at: level compression. The idea behind path compression is to replace strings of single child nodes with a single super node that has a "skip" count. The idea behind level compression is to replace full or nearly full subtrees with a super node with a "degree" count that says how many digits of the key the node decodes. There's also a 3rd approach called width compression, but I'm afraid my memory fails me and I couldn't find a description of it with quick googling.

Level compression can shorten the average path considerably, but insertion and removal algorithms get quite complicated as they need to manage the trie nodes as similarly to dynamic arrays. For the right data sets, level compressed trees can be fast. From what I remember, they're the 2nd fastest approach for storing IP routing tables, the fastest is some sort of hash trie.

6/7/2009 2:09:25 AM

I have a question regarding your implementation. What is the level of granularity that you decide to split your strings on to make the prefix tree. You could split stack as either s,t,a,c,k or st,ta,ac,ck and many other ngrams of it. Most prefix tree implementations take into account an alphabet for the language, based on this alphabet, you do the splitting.

If you were building a prefix tree implementation for python then your alphabets would be things like def, : , if , else... etc

Choosing the right alphabet makes a huge difference in building efficient prefix trees. As for your answers, you could look for PERL packages on CPAN which do longest common substring computation using trie's. You may have some luck there as most of their implementation is pretty robust.

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