This course covers the essential information that every serious programmer needs to know about algorithms and data structures, with emphasis on applications and scientific performance analysis of Java implementations. Part I covers elementary data structures, sorting, and searching algorithms. Part II focuses on graph- and string-processing algorithms. All the features of this course are available for free. It does not offer a certificate upon completion.
Ternary Search Tries
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Из курса от партнера Princeton University
Algorithms, Part II
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Tries
In this lecture we consider specialized algorithms for symbol tables with string keys. Our goal is a data structure that is as fast as hashing and even more flexible than binary search trees. We begin with multiway tries; next we consider ternary search tries. Finally, we consider character-based operations, including prefix match and longest prefix, and related applications.
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Robert SedgewickWilliam O. Baker *39 Professor of Computer Science
Computer Science
Kevin WaynePhillip Y. Goldman '86 Senior Lecturer
Computer Science
Next we're going to look at Ternary search tries, which is another data
structure that new data structure, that responds to the problem of too much
memory space used by our way tries. this is a very easy to describe and
implement data structure. And it came out of the same paper that
Jon Bentley and I wrote in the 1990s when we developed the three way Ternary
quicksort. the idea is now we're going to actually
store characters in nodes in values. but, we're not going to store whole keys
in there, just characters. but we're not going to use this idea of a
big array, where a nominal link is an implicit value of a character, actually
going to store the characters. but what we're going to do is, make each
node have three children not two, like in a binary search tree.
And not r, like in a Nr way trie, but exactly three and the three children are
for the trie corresponding to smaller keys.
The trie corresponding to trees that start with the same, character in the tri
corresponding to larger trees. And just given this, this des,
description and experience with many of the data structures that now, we've
talked about already in this course. many of you could go off and implement
this data structure, and we'll see how simple the implementations are.
but still let's work through precisely what the data structure looks like.
'cuz sometimes the recursive code looks almost too simple and kind of mass
understanding. So this is the representation of trie's,
that we started with. and all we do with Ternary search trees
is it's, it's almost like having a binary search tree representation of the
non-null links for every node. so, in this case we're going to have an
ad, an s at the root, and that depends on where your keys are inserted.
and the key idea is, the middle link coming off the root node, is a link to
the TSTs that have all the keys that start with S, with S stripped off.
now, but for the left and right links those are for keys that start with some
letter that appears before S on the left, and after S on the right.
and then say, on the left, the node for B is, if you go down this link, it's all
the keys that start with B. Otherwise the B is just to divide the
ones that are less, from the ones that are bigger, and so forth.
and again, if you [COUGH] search for a key by say, look search for SHE, so
that's going down middle links and matching keys.
Then some nodes will have values associated with them, that's the value
associated with the last letter, last character in the search key.
So, that's what the data structure looks like.
and I'd begin to describe the search algorithm, it's quite simple given the
definition of the data structure. Again, we follow links corresponding to
each character of a key. If we have a character that's less than
the character in the node, we go left, if we have one that's greater, we can go
right. and if it's equal we move to the next
character, and take the middle link. so this is an example of a search for a
SEA. Start with S, that's a match, we take the
middle link. Now the next character is E that's a
mismatch and E is less than 8, so we go to the left.
now we find a note that has E, and we didn't update our pointer into the key
character, because we didn't find a match.
So, we're still looking at the E, and now, now we can match that E, so we go
down the middle link. Now we're looking at the A in the search
key, and that's less than L, so we go to the left.
now we're still looking at the A in the search key, and that's a match and it's
the last character in key, so, we return, the value associated with the last
character in the key. So, it's the same basic algorithm, that
we used for trie's except, we just have a different way to decide whether we match
the character. We explicitly store characters in nodes.
We follow middle link on a match, and update the pointer in our key character.
Otherwise, we, we follow the left or right link, because we know that the key
is going to be found in the left or right sub-tree.
And each node has exactly three links. So here's a example of search in a TST,
again this is the example I just talked about in a dynamic form, form.
So, if S is the first character in the key, matches the S in the first node of
tree, so we take the middle link, and move on to the second character in the
key which is e. Compare that against h, it's not a match
in fact it's less, so we go left. So, now we're still looking at e and c,
and it's a match with the character in the node that we're currently processing.
So, we take the middle, and now we move to the next character in the key which is
a. And now we take a compared to l, and a is
less, so we go left, we're still looking at the a, didn't updated it, 'cuz it's
not a match, and now it is a match. and it's the last character in the key,
so we look at the value, and that's the value we return.
what about unsuccessful search? well, let's see how that works.
So, for shelter, it starts with an S, so we take the middle link, second, and move
to the second character. Second character's an h, and that's a
match. So, we take the middle link, and move to
the next character. Third character's an e and third
character, and that, that's also a match. So again, we take the middle link, and
move to the next character. the fourth character's an l, that's also
a match. So, again, we take the middle link, and
move to the next character. Now the next character is a t, which is
not a match, so we want to move to the right but moving to the right takes us to
a null link. So that means that shelter is not in the
TST and, and so we return null. Again, in every, in every step what we do
is completely well-defined. and any search for a key that in the TST,
it's going to return it's associated value.
Any search for any other key, is either going to, go out on a null link, or end
on a node that involves a mismatch. so, with that basic idea, let's take a
more detailed look in the demo, of how the tree gets constructed.
and following through this demo, will give you a pretty good feeling for how
these trees are constructed. So, we're going to associate the value 0
with a string, SHE. so, again every time we move to a new
letter, we have to create a new node, until we get to the end of the key.
And at which case, we put the, associated the value with the node that contains the
last character in the key. so, that's the starting point.
key is a sequence of characters down the middle links that [COUGH] goes to from
the root to some node. And the value is in the node that
corresponds to the last character. so, how do we put a new one in this tree
so the in this Ternary search trie. Our first character is S, so we go down
the middle. Our next character is not a match so
[COUGH] we go to the left, and it's a null link but we have a node that we want
to put there. That's the one corresponding to the
second letter, in sell's so we do that, and then we make nodes with middle links,
going until we get to the last character in sell's, and we associate that with the
value 1. So, what about SEA, let's see how that
one works. so, first character is S, so I take the
middle link, second character is e which is less than h and not a match so we move
to the left and continue looking for the e.
now this node we find the e, so we take a middle link, and move to the next node
[COUGH] next character which is a, a is less than l.
it's not a match so we go to the left, that's a null linl, and that's where we
put our a, and we associate it with the value 2.
OK, here's sh, shells so we have a match at s, go to the middle link, and go to
the next letter, match h middle link next letter.
and then we go down and add the three nodes corresponding to the last three
letters and put a three at the node corresponding to the last letter.
OK? now B Y we look at S not a match it's
less, so we're going to go to the left, that's a null link.
That's where we put our first character, and then we go down the middle link to
add the node for the last character,and then ah,associate the value for there.
and then B is a similar thing on the right.
And then go for the t and h and e, and that gets our share with the value 5.
SEA again, it's associative, so we do the search, so S is a match.
So we go down the middle link, and move to the next letter.
E is less than h so we go to left, and keep looking for e.
Now we find e is a match, so we go down the middle link, and move to the next
letter. that's a, which is less than l, so we go
to the left to look for the a, and there we find it.
and we're [COUGH] building an associate symbol table, so we update and overwrite
the old value with the new value, 6. S, H, O, R, E.
we match the S, match the h, now we're on O, that does not match e, so we go to the
right, and keep looking for O. Now, we don't find it, so we create a new
node for O, R, E, and then put the value 7 in the values associated with the last
character in that key. so, that's the construction of a Ternary
search tree containing eight keys. Let's take a look at a slightly bigger
Ternary search trie. so here's our example with three letter
words. now for that example, we didn't show all
the null links, but there's lots of null links, 26 null links in each leaf for
this 26 way trie. And there's other null links higher in
the tree, and actually counting up there's over a thousand null links in
this tiny 26 way trie. But in a TST, it's got about the same
number of nodes it's actually got more nodes, because it's got one for each
character. And the other ones correspond to links,
but not that many more nodes. But the key thing is, it has many fewer
null links, because there's only three links per node.
And this effect is obviously going to be much more dramatic when R is much bigger.
like a 256 way trie or even a unicode 65,000 way trie.
that's the key benefit of TSTs, that and they're much more space-efficient, then
our way trie's for large R. so, let's take a look at the
implementation in Java, and then we'll look at the code, and then we'll talk
some more about performance. oh, the representation in Java is very
straightforward, as I said many of you could code this up, just from the
description. what does a node have to have?
It's going to have a value, it's going to have a character.
it's going to have references to left, middle, and right TSTs.
and that's what it's built from. so whereas the standard trie
representation has a ray of links with characters implicit.
TST has explicit characters with only three links per node.
so that's TST representation and then given the representation the code is
follows almost immediately. with our standard setup if we're supposed
to, now we use, we call a recursive routine, giving the root as first
argument. And give the tree returned going to give
a reference that back to the root. so our recursive routine takes a node and
returns a node and for most of the time, it, it doesn't do much.
But when it can do node, is created this this code is effective, because any link
we go down, we replace with a link that we got back, our standard setup.
so, the recursive port routine if we're that character d, we pull out the d
character. if once we've done that, if our node is
null, we create a new node and give it that character.
and now we test our character in our search key, against the character in the
given node. Either the one we just created, or the
one we happen to be on. if if it's equal, it will fall through to
test if we are on the last key. If we're not, if, if we're on the last
character in the key we just reset the value.
if its equal and we're not on the last character in the key, we go down the
middle link. and we replace that link with what we get
by going down the middle link by moving to the next character in the key.
otherwise, we go either down the left or right link under the same circumstances,
without moving to the next character in the key.
so that's extremely straightforward code for put in Ternary search trie's.
and again, get is even simpler. it's a similar set up that we've used
several times before. Use a recursive routine that returns a
node, and we pull our value out of that node.
we don't have to cast in this case, cause we're not running arrays, and our nodes
just has the value type in it. and if we get a null node back, yea, we
return null that its not there, the recursive routine will pull out the
character if its less than our if our search character is less we go to the
left. And if its greater we go to the right,
and if its equal we and we're not at the end of the key, we go down to the middle
and we move to the end of the key. If its equal and we are at the end of the
key, we return the node. And even that node, is going to contain a
value or it's not, but that's what we'e return.
so, extremely straightforward implementation of both get and put for
Ternary search drives. so this, adds this line to our table,
where, the amount of space taken for Ternary search trie's is just three
lengths you know, plus the value character in each node.
so similar to what red-black BST and hashes would use, so no space
disadvantage. the determining the cost of search hits
and search miss, these values are given for random keys, and they're the subject
of deep analysis. but it's not too many.
Le, Let's, let's put it that way. And our experimental results bear that
out. So, for, for a search miss in a Ternary
search trie, you don't have to look at the whole key.
Usually, you look at maybe fewer characters than than the whole key, and
actually these results are, are for typical, or random.
but actually TSD's really shine, in the case where data is not random.
they very well adapt to the data. like text written in English language, is
not really language really random. And you can see that Ternary search trees
actually out perform both hashing and red black BSTs for our benchmark, which is
deduping these big text files. [COUGH] now you, you can go ahead and try
to get worst case guarantees. By using rotations to balance the the the
internal binary search tree's corresponding to each character in, in
TSTs. although that's probably normally not
worth the trouble. but the bottom line is that TST is at
least it's as fast as hashing for string keys and it's it's space efficient for
sure. And it's pretty easy to speed it up.
the idea is to speed it up, that works extremely well in practice is to, use our
way of branching, at the be, at the top of the TST.
so rather than fool around with the first, couple of characters, simply do a
big branching at the root. You can do r way branching at the root of
26, but in practice, it's pr, proven it works fine to do, let's say, r squared
way branching at the root. and immediately take care of the first
two characters, which is the common case, and then get down to small TSTs.
and there's variations of this, that you might imagine, but this is a really
simple one that almost always works. you have to deal especially with one and
two letter words. and we'll leave that for exercise.
but with that change now we get a, an algorithm that's is definitely faster
than hashing in red-black BSTs for our benchmarks.
and that'll turn out to be the case, for many applications.
So, just to summarize what are the trade offs between TSTs and hashing?
for hashing you have to examine the entire key, in order to compute the hash
function. we talked about examples, where people
try to skip doing that, and wind up with performance problems.
Not much difference between a hit and a miss in a hashing, and there's a lot of,
of reliance and maybe difficulty of implementation ah,depending on the hash
function. and also, hashing is really only good for
search and insert. the other kinds of operations that we'd
like to perform in symbol table when there's order in the keys is not
supported by hashing. with TSTs, it only works for strings or
if you have keys that are numbers, then you can rework them into strings, or
string like things. And that's that's a restriction but does
cover a huge number of applications. it, it only examines the key characters
that it needs to, and in particular, a search miss might involve only a few
characters. So, in typical applications you'll get
sub-linear performance in TSTs. You can get searches done without looking
at the whole key. the other thing is there's other, the
ordered symbol table operations you can, you know easily support with TSTs.
just by extending, what we did for binary search tree.
and even more important there's other interesting operations that we can add
for string keys. that are very useful, and we'll see
applications, and implementations of these, in just a minute.
so the bottom line is that this data, is a relatively new data structure.
but, its better than the classic ones for important applications.
Its faster than hashing particular for search misses.
and its got more flexibility even than red black BSTs, and that's what we will
talk about soon, but that's an introduction to TSTs.
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