# Five Letter Words: Part 5: The Try Trie Tree

Posted in Computer Science

In Volume 4 Fascicle 0 of Donald Knuth's Art of Computer Programming, Knuth introduces a tool for exploring concepts in graph theory: the five-letter words. This is a collection of 5,757 five-letter words compiled by Knuth and useful in exploring ways of constructing efficient algorithms.

The word list is large enough that an $$O(N^2)$$ algorithm will take a solid chunk of CPU time, so there's a definite incentive to think carefully about implementation.

Knuth introduces a list of five-letter words, as well as associated exercises utilizing techniques from dynamic programming and graph theory, among other topics.

We have covered this topic before in prior blog posts:

and a recent addendum to Part 1:

We continue our coverage in this blog post with a newer problem, one that is rated by Knuth at 26, on his scale of 0 to 50:

00  Immediate
10  Simple (1 minute)
20  Medium (quarter hour)
30  Moderately hard
40  Term project
50  Research problem


(from Volume 1, Notes on the Exercises.)

Here's the list of words if you want to play along.

Link to the Stanford Graph Base.

Visit Five Letter Words on the charlesreid1.com wiki for details.

## Introduction to the Try Trie Tree Problem

In this blog post, we'll cover Exercise 35 of Volume 4, Fascicle of Donald Knuth's Art of Computer Programming.

The problem is as follows:

Sixteen well-chosen elements of WORDS(1000) lead to the branching pattern (figure), which is a complete binary trie of words that begin with the letter s. But there's no such pattern of words beginning with a, even if we consider the full collection WORDS(5757).

What letters of the alphabet can be used as the starting letter of sixteen words that form a complete binary trie within WORDS(n), given n?

For the benefit of those without the book, here is an attempt to represent the trie that Knuth includes in the exercise:

                     s

h                 t

e       o         a       e

e   l   r   w     l   r   a   e

sheep             stalk
sheet             stall

shelf             stars
shell             start

shore             steal
short             steam

shown             steel
shows             steep


To answer the question, of whether a complete binary trie can be completed for a given letter, given a set of n words, we construct a "try trie tree," which is a tree data structure that greedily builds a trie with as many branches as possible.

The full trie of $$26^4+1 = 456,977$$ nodes would be expensive to assemble in full, for each starting letter. Instead we use the word list to build up a tree of possible candidate branches for the trie. Once we've constructed all possible branches using the faster but less precise method, we verify that each candidate branch we have constructed either meets our criteria (can be included as a branch in a complete binary trie), or is pruned.

## The Try Trie Tree

To solve this problem, we define a TryTrieTree class that holds the nodes and links that make up our tree. We define some methods for it to perform the assembly and verification operations described below, then assemble one try trie tree for each letter of the alphabet to come up with an answer to the exercise.

## Constructing the Try Trie Tree

The construction procedure for the try trie tree proceeds in two steps:

Step 1 is to assemble a tree, from the top down, by searching the entire space of $$456,977$$ nodes and marking particular nodes and paths on this tree as candidates for the final perfect binary trie.

Step 2 is to revisit the candidate branches, proceeding from the bottom up, and determine if the candidate branches do, in fact, have enough sibling nodes and word matches to form a complete branch in a perfect binary trie.

We start Step 1 at the root node (the starting letter) and proceed from the root down, going level by level.

### Checking for Minimum Number of Matching Words

Step 1 proceeds from the top down and marks branches that are candidates to end up in the final perfect binary trie.

At each level of the trie, we count the number of words in the overall word set that have a prefix matching the prefix corresponding to that node.

For example, the trie node b on the path s-a-b would yield four words:

saber
sable
sabre
sabra


If enough words match, that branch of the trie is a possible candidate to end up in the perfect binary trie (but may be trimmed in Step 2).

Example: If we are assembling the complete trie for s given by the author in the exercise, we can verify that there are at least 16 words that begin with the letter s. We would then verify that there are at least 8 words that begin with sa, which there are. Then we would verify that there are at least 4 words that begin with saa, which there are not, so we would move on to verifying that there are at least 4 words that begin with sab, which there are. We would proceed in this fashion until we had assembled a candidate trie branch, s-a-b-r (which contains two words, sabra and sabre). For Step 1, we keep s-a-b-r as a candidate branch. (We will see in Step 2 that this branch will get trimmed.)

At each level of the trie, we apply the procedure: - At level 1, we require a minimum of $$2^{5-1} = 16$$ words. - At level 2, we require a minimum of $$2^{5-2} = 8$$ words. - At level 3, we require a minimum of $$2^{5-3} = 4$$ words. - At level 4, we require a minimum of $$2^{5-4} = 2$$ words.

NOTE: We are not explicitly constructing the trie, so we don't need to assemble the word leaves.

### Assemble Method

See the Try Trie Tree Code section for the code for the public and private assembly methods.

To peform the assembly of all possible branches of the try trie, we use the assemble() method. This is a public method that starts a recursive call to a private method _assemble().

We are given a starting letter (in the example given by the author, the starting letter is "s"). We explore every possible prefix that starts with the root letter, "sa", "sb", "sc", "sd", and so on.

For each of those possible prefixes, we explore every possible third letter, "saa", "sab", and so on, and then once more in a fourth step, "saaa", "saab", ..., for a total of $$26^4 = 456,976$$ iterations (checks for existence of words starting with a given substring).

A substantial number of these checks will do nothing - from the fact that

$$\frac{5757}{456976} \sim 5e3/5e5 \sim 0.01$$

we know that the maximum number of loops that would actually lead to a branch being added will be 1% of that $$26^4$$ total.

We also know that any efforts to speed up this program should focus on the way we are checking the number of words that start with a given substring - since that's where we'll spend most of our time.

The recursive assembly method takes a prefix string input (which maps to a location in the tree), and it explores all 26 possible children of that prefix (location in the trie).

When a leaf node is reached, at the fourth level, it represents the longest prefix in our trie. This is the base case of the recursive assembly function; the recursive function terminates at a fixed depth of 4.

### Verifying Branches and Bubbling Up Counts

However, as we noted, the counts we are using above in Step 1 are just approximations (checking there are a minimum number of words with a given prefix). There is no way to guarantee that a trie branch can be used in the final complete perfect binary trie until all child leaf nodes have been visited.

This is where Step 2 comes in. In Step 2 we perform a pre-order depth-first traversal of the tree, visiting the leaf nodes first and proceeding from the bottom up. The number of words matching the prefix of each leaf node must be 2, to keep the leaf node.

We then proceed up the tree, level by level, and at each level we require that each node have at least 2 "large" children. This is a recursive definition - for a child to be "large", it must itself have 2 "large" children, or (if it is a leaf node) it must have at least 2 words that match the 4-letter prefix.

In our TryTrieTree, a complete binary trie is only possible if each node at each level has two or more children that are "large enough", where "large enough" means that either (a) both child nodes have two or more children that are "large enough", or (b) if we are at a leaf node (representing 4 characters), and there are two or more words that begin with the 4 characters corresponding to this trie node.

Let's go through an example.

Continuing with the example above for s, we assembled the branch s-a-b-r, which contains the minimum two words required. However, s-a-b is not a common enough prefix! The only child of s-a-b with two or more words matching is s-a-b-r, which means we can't form a complete binary trie using this s-a-b branch.

We call this procedure a "bubble up" procedure, since it is bottom-up.

### Bubble Up Method

See the Try Trie Tree Code section for the code for the public and private bubble up methods.

Similar to the assembly method, our bubble up method is also a recursive method, performing a depth-first pre-order traversal. This ensures we reach leaf nodes before beginning our task, and that counts proceed from bottom-up.

## Try Trie Tree Code

Below we go through some of the code for the Try Trie Tree problem.

### Try Trie Trie Class

When dealing with trees, it's always a safe bet that we'll need a Node class, so we start with a utility class for tree nodes:

class Node(object):
def __init__(self, letter, count=0):
self.letter = letter
self.count = count
self.children = []
self.parent = None


The TryTrieTree class has a constructor that starts with an empty root. The tree should also contain a pointer to the original word set, so that we can reference it in later methods where needed.

class TryTrieTree(object):
def __init__(self,words):
self.root = None
self.words = words


In the final class we defined a __str__() method to create a string representation of the TryTrieTree, but we will skip that for now.

Next we have a method to set the root to a given Node:

    def set_root(self,root_letter):
self.root = Node(root_letter)


Additionally, we have two utility methods that help us navigate between locations in the tree and the corresponding string prefixes. These two methods convert between string prefixes (like s-a-b-r) and locations in the trie (like the r trie node at the end of the path s-a-b-r):

• get_prefix_from_node() (utility method): given a Node in the trie, return the string prefix that would lead to that Node.

• get_node_from_prefix() (utility method): given a string prefix, return the Node in the trie that corresponds to the given string prefix. Return None if no such Node exists.

These methods are given below.

First, to convert a particular node location to a string prefix, we use the parent pointer of each node to traverse up the tree and assemble the corresponding prefix string from the path (so that traversing from b to a to the root s would yield sab):

    def get_prefix_from_node(self,node):
"""Given a node in the trie,
return the string prefix that
"""
if node==None:
return ""
elif node==self.root:
return ""
else:
prefix = ""
while node.parent != None:
node = node.parent
prefix = node.letter + prefix
return prefix


and the reverse, to convert a string prefix into a location in the trie:

    def get_node_from_prefix(self,prefix):
"""Given a string prefix,
return the node that represents
the tail end of that sequence
of letters in this trie. Return
None if the path does not exist.
"""
assert self.root!=None

if prefix=='':
return None

assert prefix[0]==self.root.letter

# Base case
if len(prefix)==1:
return self.root

# Recursive case
parent_prefix, suffix = prefix[:len(prefix)-1],prefix[len(prefix)-1]
parent = self.get_node_from_prefix(parent_prefix)
for child in parent.children:
if child.letter == suffix:
return child

# We know this will end because we handle
# the base case of prefix="", and prefix
# is cut down by one letter each iteration.


### Code for Assembling the Tree

We assemble the tree using a private recursive method. Here is how that looks (again, these methods are defined on the TryTrieTree class):

    def assemble(self):
"""Assemble the trie from the set of words
passed to the constructor.
"""
assert self.root!=None

words = self.words

prefix = ''
candidate = self.root.letter
self._assemble(prefix,candidate,words)


In the private recursive method, we assemble the branches of the tree, only checking to make sure each branch has the minimum number of words required.

At the start of each assemble method, we whittle the set of words down to only the words that start with the prefix for the given node. This trick uses a little extra space but the payoff is avoiding searching the entire word set for each node to count the number of words matching a given prefix. If a node's parent is s-a-b and we have already done the work of filtering all words starting with sab, there is no need to repeat that work when finding and filtering all words that start with sabr.

    def _assemble(self,prefix,candidate,words):
"""Recursive private method called by assemble().
"""
prefix_depth = len(prefix)
candidate_depth = prefix_depth+1

ppc = prefix+candidate
words_with_candidate = [w for w in words if w[:candidate_depth]==ppc]


Next lines are the checks to ensure we have the minimum number of words required to form a candidate branch in the trie.

If we do, we will create a new child node for that branch and recurse by calling assemble on it.

Of course, we have to check for the base case, which in this scenario checks when we have reached the fixed trie depth of 4.

        min_branches_req = int(math.pow(2,5-candidate_depth))
max_number_branches = len(words_with_candidate)

# If we exceed the minimum number of
# as a new node on the trie.
if max_number_branches >= min_branches_req:

parent = self.get_node_from_prefix(prefix)

# If we are looking at the root node,
if prefix=='':
# parent will be None.
# In this case don't worry about
# creating new child or introducing
# parent and child, b/c the "new child"
# is the root (already exists).
pass

else:
# Otherwise, create the new child,
# and introduce the parent & child.
new_child = Node(candidate)
new_child.parent = parent
parent.children.append(new_child)

# Base case
if candidate_depth==4:
new_child.count = max_number_branches
return

# Recursive case
for new_candidate in ALPHABET:
new_prefix = prefix + candidate
self._assemble(new_prefix,new_candidate,words_with_candidate)

# otherwise, we don't have enough
# branches to continue downward,
# so stop here and do nothing.
return


### Code for Bubbling Up Large Children Counts

These are a little shorter and simpler than the assembly method above:

    def bubble_up(self):
"""Do a depth-first traversal of the
entire trytrietree, pruning as we go.
This is a pre-order traversal,
meaning we traverse children first,
then the parents, so we always
know the counts of children
(or we are on a leaf node).
"""
self._bubble_up(self.root)

def _bubble_up(self,node):
"""Pre-order depth-first traversal
starting at the leaf nodes and proceeding
upwards.
"""
if len(node.children)==0:
# Base case
# Leaf nodes already have counts
# Do nothing
return

else:
# Recursive case
# Pre-order traversal: visit/bubble up children first
for child in node.children:
self._bubble_up(child)

# Now that we've completed leaf node counts, we can do interior node counts.
# Interior node counts are equal to number of large (>=2) children.
large_children = [child for child in node.children if child.count >= 2]
node.count = len(large_children)


You can see how we converted the definition of "large children" into a rule above - we use the recursive case of the "large children" definition in the recursive case, and we use the base case of the "large children definition" (for leaf nodes) when we are on the base case.

Also note that each leaf node was initialized with the number of words that start with the corresponding 4-letter prefix (that was done in the assembly method), but we could just as easily do it in the base case, as the leaf nodes are the base case.

## Wrap it in a Bow

We can add some extra wrapping around our class, and call each of the methods in order for the various letters of the alphabet.

Below, we process an input argument n (which is the size of the wordlist, 5757, if the user does not specify n). It then creates a TryTrieTree for each letter, and determines if a complete binary trie can be constructed. Finally, it prints a summary of the results.

#!/usr/bin/env python
from get_words import get_words
import sys
import math

"""
tries.py

Donald Knuth, Art of Computer Programming, Volume 4 Fascicle 0
Exercise #35

Problem:
What letters of the alphabet can be used
as the starting letter of sixteen words that
form a complete binary trie within
WORDS(n), given n?
"""

ALPHABET = "abcdefghijklmnopqrstuvwxyz"
FIVE = 5

class Node(object):
...

class TryTrieTree(object):
...

def trie_search(n, verbose=False):

words = get_words()
words = words[:n]

perfect_count = 0
imperfect_count = 0
for letter in ALPHABET:

tree = TryTrieTree(words)
tree.set_root(letter)
tree.assemble()
tree.bubble_up()
#print(tree)

if tree.root.count >= 2:

if verbose:
print("The letter {0:s} has a perfect binary trie in WORDS({1:d}).".format(
letter, n))
perfect_count += 1

else:

if verbose:
print("The letter {0:s} has no perfect binary trie in WORDS({1:d}).".format(
letter, n))
imperfect_count += 1

if verbose:
print("")
print("Perfect count: {:d}".format(perfect_count))
print("Imperfect count: {:d}".format(imperfect_count))

return perfect_count, imperfect_count

def trie_table():
"""Compute and print a table of
number of words n versus number of
perfect tries formed.
"""
print("%8s\t%8s"%("n","perfect tries"))

ns = range(1000,5757,500)
for n in ns:
p,i = trie_search(n)
print("%8d\t%8d"%(n,p))

n = 5757
p,i = trie_search(n)
print("%8d\t%8d"%(n,p))

if __name__=="__main__":
if len(sys.argv)<2:
n = 5757
else:
n = int(sys.argv[1])
if n > 5757:
n = 5757

_,_ = trie_search(n, verbose=True)

#trie_table()


## Output

When we run with n = 1000, we can see that s is the only letter that forms a perfect binary trie for that value of n:

$python tries.py 1000 The letter a has no perfect binary trie in WORDS(1000). The letter b has no perfect binary trie in WORDS(1000). The letter c has no perfect binary trie in WORDS(1000). The letter d has no perfect binary trie in WORDS(1000). The letter e has no perfect binary trie in WORDS(1000). The letter f has no perfect binary trie in WORDS(1000). The letter g has no perfect binary trie in WORDS(1000). The letter h has no perfect binary trie in WORDS(1000). The letter i has no perfect binary trie in WORDS(1000). The letter j has no perfect binary trie in WORDS(1000). The letter k has no perfect binary trie in WORDS(1000). The letter l has no perfect binary trie in WORDS(1000). The letter m has no perfect binary trie in WORDS(1000). The letter n has no perfect binary trie in WORDS(1000). The letter o has no perfect binary trie in WORDS(1000). The letter p has no perfect binary trie in WORDS(1000). The letter q has no perfect binary trie in WORDS(1000). The letter r has no perfect binary trie in WORDS(1000). The letter s has a perfect binary trie in WORDS(1000). The letter t has no perfect binary trie in WORDS(1000). The letter u has no perfect binary trie in WORDS(1000). The letter v has no perfect binary trie in WORDS(1000). The letter w has no perfect binary trie in WORDS(1000). The letter x has no perfect binary trie in WORDS(1000). The letter y has no perfect binary trie in WORDS(1000). The letter z has no perfect binary trie in WORDS(1000). Perfect count: 1 Imperfect count: 25  In fact, 978 is the smallest number of words to find any perfect tries: $ python tries.py 978
The letter a has no perfect binary trie in WORDS(978).
The letter b has no perfect binary trie in WORDS(978).
The letter c has no perfect binary trie in WORDS(978).
The letter d has no perfect binary trie in WORDS(978).
The letter e has no perfect binary trie in WORDS(978).
The letter f has no perfect binary trie in WORDS(978).
The letter g has no perfect binary trie in WORDS(978).
The letter h has no perfect binary trie in WORDS(978).
The letter i has no perfect binary trie in WORDS(978).
The letter j has no perfect binary trie in WORDS(978).
The letter k has no perfect binary trie in WORDS(978).
The letter l has no perfect binary trie in WORDS(978).
The letter m has no perfect binary trie in WORDS(978).
The letter n has no perfect binary trie in WORDS(978).
The letter o has no perfect binary trie in WORDS(978).
The letter p has no perfect binary trie in WORDS(978).
The letter q has no perfect binary trie in WORDS(978).
The letter r has no perfect binary trie in WORDS(978).
The letter s has a perfect binary trie in WORDS(978).
The letter t has no perfect binary trie in WORDS(978).
The letter u has no perfect binary trie in WORDS(978).
The letter v has no perfect binary trie in WORDS(978).
The letter w has no perfect binary trie in WORDS(978).
The letter x has no perfect binary trie in WORDS(978).
The letter y has no perfect binary trie in WORDS(978).
The letter z has no perfect binary trie in WORDS(978).

Perfect count: 1
Imperfect count: 25


Running with the full 5757 words leads to 11 more perfect tries:

\$ python tries.py 5757
The letter a has no perfect binary trie in WORDS(5757).
The letter b has a perfect binary trie in WORDS(5757).
The letter c has a perfect binary trie in WORDS(5757).
The letter d has a perfect binary trie in WORDS(5757).
The letter e has no perfect binary trie in WORDS(5757).
The letter f has a perfect binary trie in WORDS(5757).
The letter g has no perfect binary trie in WORDS(5757).
The letter h has a perfect binary trie in WORDS(5757).
The letter i has no perfect binary trie in WORDS(5757).
The letter j has no perfect binary trie in WORDS(5757).
The letter k has no perfect binary trie in WORDS(5757).
The letter l has a perfect binary trie in WORDS(5757).
The letter m has a perfect binary trie in WORDS(5757).
The letter n has no perfect binary trie in WORDS(5757).
The letter o has no perfect binary trie in WORDS(5757).
The letter p has a perfect binary trie in WORDS(5757).
The letter q has no perfect binary trie in WORDS(5757).
The letter r has a perfect binary trie in WORDS(5757).
The letter s has a perfect binary trie in WORDS(5757).
The letter t has a perfect binary trie in WORDS(5757).
The letter u has no perfect binary trie in WORDS(5757).
The letter v has no perfect binary trie in WORDS(5757).
The letter w has a perfect binary trie in WORDS(5757).
The letter x has no perfect binary trie in WORDS(5757).
The letter y has no perfect binary trie in WORDS(5757).
The letter z has no perfect binary trie in WORDS(5757).

Perfect count: 12
Imperfect count: 14


If we assemble a table of number of five letter words n versus number of perfect tries formed, nearly half show up only after we include 4,500 words.

       n    perfect tries
1000           1
1500           1
2000           1
2500           1
3000           3
3500           3
4000           4
4500           6
5000          11
5500          12
5757          12