Implement Trie II (Prefix Tree)
For this problem, we need to maintain a trie structure where each node in the trie represents a character from a string. Every node should also have a count variable to store how many times a string ends at that node. This will help in implementing the countWordsEqualTo
and erase
functions. Additionally, every node should have another count variable to store how many strings pass through that node, which will aid in the countWordsStartingWith
function.
Data Structure
 A Trie Node structure which contains:
 A dictionary called
children
to store the next characters.  A count variable
endOfWord
to keep track of the number of times a word ends at this node.  A count variable
passingWords
to keep track of the number of words passing through this node.
 A dictionary called
Method Implementations
insert: Traverse the trie using the characters of the word. If a character is not present, create a new node. Finally, increment the
endOfWord
count of the last character of the word.countWordsEqualTo: Traverse the trie using the characters of the word. If any character is not found, return 0. Otherwise, return the
endOfWord
count of the last character of the word.countWordsStartingWith: Traverse the trie using the characters of the prefix. If any character is not found, return 0. Otherwise, return the
passingWords
count of the last character of the prefix.erase: Traverse the trie using the characters of the word and decrease the
endOfWord
count andpassingWords
count for each node in the path.
Here’s the code for the Trie:


The provided data structure efficiently maintains and manipulates the strings while keeping track of their count and prefixes.
Identifying Problem Isomorphism
“Implement Trie II (Prefix Tree)” has an approximate isomorphism “Implement HashMap”. Both are about implementing data structures that support specific operations, though the data structures themselves are different.
In “Implement Trie II (Prefix Tree)”, you’re asked to implement a trie (or prefix tree) which supports the operations of insertion, deletion, search, and prefix counting. In “Implement HashMap”, you’re asked to implement a hash map that supports the operations of insertion, deletion, and searching.
While both are data structures that store and retrieve information, a Trie is a treelike data structure that is used mainly in applications of string manipulations, where as a HashMap is a widelyused data structure that uses a hash function to map keys to their associated values.
Although the implementations of these two data structures are quite different, the basic problem structure is analogous: implementing a data structure that supports specific operations. However, it should be noted that implementing a Trie generally involves more complex tree operations, while implementing a HashMap primarily involves understanding hashing and handling possible collisions, which could be simpler depending on one’s familiarity with these concepts. This makes “Implement Trie II (Prefix Tree)” a more complex problem compared to “Implement HashMap”.
This is an approximate isomorphism because the actual data structures and their internal mechanisms are quite different. They’re isomorphic in the sense of the problem structure and requirements, but not in terms of the specific details.
10 Prerequisite LeetCode Problems
This is an application of Trie (a type of search tree), which is widely used in storing and retrieving keys in a dataset of strings. This involves a deep understanding of tree data structures and string manipulation.
To tackle this problem, you should have a good understanding of basic data structures (like arrays, strings, linked list, trees), recursion, and objectoriented programming.
Here are some simpler problems to prepare:
LeetCode 208: Implement Trie (Prefix Tree): This problem is about implementing a simple Trie (without count). It is the most basic and important preparation for the current problem.
LeetCode 211: Design Add and Search Words Data Structure: This problem will help you understand how to implement a Trie to perform a specific task, such as adding and searching words.
LeetCode 677: Map Sum Pairs: This problem will help you understand how to use Trie in conjunction with other data structures.
LeetCode 139: Word Break: This problem doesn’t explicitly use a Trie, but the string parsing logic is similar to how you’d build and traverse a Trie.
LeetCode 212: Word Search II: This problem involves searching for multiple words in a grid. It gives more practice on Trie traversal.
LeetCode 79: Word Search: This problem involves searching for a single word in a grid. It is simpler than problem 212.
LeetCode 14: Longest Common Prefix: This problem is about prefixes which is a basic operation for Trie.
LeetCode 720: Longest Word in Dictionary: This problem requires you to use a Trie to solve a more complex problem, which is good preparation for this problem.
LeetCode 116: Populating Next Right Pointers in Each Node: This problem teaches you how to handle nodes in a tree, which is essential when working with Tries.
LeetCode 102: Binary Tree Level Order Traversal: This problem doesn’t involve a Trie, but it gives essential practice on tree traversal, which is useful for understanding Trie operations.
These cover Trie data structure, how to use it, and various operations you can perform on it.
Clarification Questions
Here are some potential clarification questions that could be asked about this problem:
What characters or encoding can the input strings contain  only ASCII, unicode, etc?
How should duplicate insertions be handled  count them separately or overwrite?
For count of prefixes, should overlapping prefixes also be counted? e.g. “app” and “apple” for input “apple”.
Should the trie handle prefix matching for search in addition to counting prefixes?
Is casesensitivity important for search, insert, delete operations?
What should be returned if a delete or count operation is called on a nonexistent string?
Are there limits on the maximum lengths of inserted strings or prefixes to count?
What are the most frequent operations  insert, delete, search? This may impact structure.
How should memory usage be optimized  minimize footprint or maximize speed?
Can we assume the inputs are cleaned/validated before being passed to the trie?
Are there latency requirements for supported operations?
Asking questions like these would help clarify ambiguity in expected behavior, usage patterns, edge cases, and constraints. This enables designing an optimized trie suited for the problem’s domain.
Problem Analysis and Key Insights
Here are some key insights gained from analyzing the problem statement:
The trie supports dynamic insertion and deletion of strings indicating a mutable structure.
Counting exact string matches and prefixes points to efficient search being a key requirement.
The nature of supported operations hints at prioritizing fast lookups and edits over space optimization.
The fixed alphabet of lowercase English letters may allow optimizations like character encoding.
Erasing strings that are guaranteed to exist simplifies deletion logic compared to a general case.
Reasonable limits on string lengths and total operations prevents brute force solutions.
Counting requirements mean storing frequency information at trie nodes.
Support for prefixes points to compact trie representations like compressed or radix tries.
The problem is focused on implementing the data structure and operations rather than applying it to an external problem.
Overall, these insights around mutable structure, fast operations, language assumptions, frequency tracking and compact representations help narrow down optimal trie variations and customizations to fit the problem.
Problem Boundary
Based on the problem statement and analysis, the scope of this problem is:
Input space  Strings of lowercase English letters up to 2000 characters
Output  Results of requested operations like count of strings or prefixes
Methods  Insert, delete, count of exact strings and prefixes
Objective  Implement a trie supporting the required operations efficiently
Nongoals  Using the trie for any specific application, parse input strings, handle invalid input
So in summary, the scope is limited to:
Implementing the trie data structure itself
Supporting insert, delete, search, count operations
Handling lowercase English string inputs
Returning outputs of operations as required
The focus is on the trie implementation and immediate operations rather than external usage for a specific application. Input handling, output formatting etc. are out of scope.
Here are some ways we can establish boundaries for this trie problem:
Input:
 Strings containing only lowercase English letters
 Maximum string length 2000
 Total operations <= 30,000
 Inserted strings guaranteed to exist when deleting
Functions:
 insert(string)  Add string to trie
 countWordsEqualTo(string)  Return count of exact string
 countWordsStartingWith(prefix)  Return count of prefix matches
 erase(string)  Delete string from trie
Output:
 Return values from supported operations
 No other output requirements
Performance:
 No specific time or space complexity constraints
State:
 Trie structure modified by operations
 No persistent state across operations
So in summary, the fixed input format, restricted operations, focused output requirements, and lack of strict performance constraints clearly define the scope and boundaries. The problem is limited to implementing the mutable trie structure and specified operations.
Problem Classification
This problem involves implementing a trie data structure to efficiently store and retrieve string keys. It falls under the domain of strings, data structures, and algorithms.
What:
 A trie that stores strings
 Inserting strings into the trie
 Counting frequency of exact strings
 Counting strings with a prefix
 Erasing strings from the trie
 Managing the trie structure and operations
Based on these aspects, we can categorize this problem as:
 Data structures  Implements a trie with desired methods
 String operations  Inserting, searching, erasing string keys
 Frequency counting  Counting occurrences of exact strings or prefixes
 Dynamic structure modification  Supporting inserts and deletes
So in summary, this is a data structures problem focused on implementing a trie with associated operations like search, frequency counting, and dynamic edits. It relies heavily on concepts of tries, strings, algorithms, and efficient look up. The core challenge is optimizing the implementation for fast performance across use cases.
Distilling the Problem to Its Core Elements
This problem involves implementing a trie data structure to efficiently store, retrieve and manipulate a set of string keys.
The fundamental concept is using a prefix tree to store strings in a way that facilitates rapid lookups and counts based on string prefixes.
The simplest description is that we want to store a bunch of text strings efficiently to allow quick searching and counting of strings with common prefixes.
The core problem is designing data structures and algorithms to store a set of strings compactly while enabling efficient prefix queries. We can simplify it to storing strings, adding/removing strings, and counting strings with given prefixes.
The key components are:
 The trie node structure to store characters
 Methods to insert strings into the trie
 Traversal logic to count nodes with given prefixes
 Erasing nodes/edges when removing strings
The minimal operations needed are:
Inserting strings: Add nodes for each character
Counting prefixes: Traverse nodes matching prefix
Removing strings: Delete nodes/edges
So in summary, it involves compactly storing string keys using a prefix tree to achieve efficient insertion, prefix counting/lookups, and deletion  the essence is leveraging the tree structure for scalable string manipulation based on shared prefixes.
Visual Model of the Problem
Here are some ways to visualize the problem statement for implementing a trie (prefix tree) data structure:
Draw a sample trie with nodes and edges representing some inserted words. This illustrates the tree structure based on shared prefixes.
Show searches for words and prefixes on the sample trie. Animating the traversal path in the diagram helps understand lookup logic.
Depict insertion of a new word into the trie stepbystep, adding nodes and edges for each letter.
Similarly animate removal of a word, backtracking and deleting nodes.
Use different colors to highlight all nodes and edges involved when searching for a prefix.
Show counts for words and prefixes visually using grouped tally marks or numbers overlaid on trie branches.
Visualize memory usage with boxes of different sizes at each node representing storage of references vs individual characters.
Compare trie structure to alternatives like linked lists to highlight compactness and search speed.
Use real text corpus examples to show realistic tries with word frequencies, prefixes, etc.
These types of visualizations leverage imagery to clarify aspects of tries like tree structure, prefix sharing, operations like insertion and lookup, and space/time efficiencies  making the abstract concrete.
Problem Restatement
Here’s how I would paraphrase the problem statement in my own words:
The goal is to implement a trie (prefix tree) data structure to store strings efficiently and allow different operations on the stored strings.
Specifically, we need to design the trie node structure and write methods to:
 Insert a new string into the trie
 Count how many times a given string occurs in the trie
 Count how many strings have a given prefix
 Remove a string from the trie
The trie should store strings compactly by factoring out common prefixes between strings as much as possible.
We want efficient algorithms for the above operations. In particular, prefixes queries and inserts/deletes should be fast.
The data consists only of lowercase English letters and strings up to 2000 letters long.
Overall operations are limited to 30,000 to ensure the solution scales reasonably.
We can assume all erase operations refer to existing strings in the trie.
The key points are to recognize this involves:
 Storing strings efficiently in a trie structure
 Implementing key operations like insert, count, erase
 Optimizing for fast prefix queries in particular
 Constraining the data and overall operation count for scalability
Let me know if I’m missing or misunderstanding any key aspects of the problem based on this paraphrasing!
Abstract Representation of the Problem
Here’s one way we could formulate an abstract representation of this trie problem:
We have a set S of strings over some alphabet Σ.
We want to construct a data structure T that stores the strings in S with these operations:
INSERT(T, s)  Insert string s into structure T
COUNTEQ(T, s)  Return number of copies of s stored in T
COUNTPRE(T, p)  Return number of strings in T with prefix p
DELETE(T, s)  Remove occurrences of string s from T
The structure T must have these properties:
Compactness  T should store strings in a compact form by maximizing prefix sharing
Efficiency  INSERT and DELETE should operate in O(L) time where L is string length
Prefix search  COUNTPRE must operate in O(P) time where P is prefix length
By abstracting away the specifics of a trie and using more general language around string sets, operations, and efficiency properties, we can focus the problem on the core ideas like compact storage, prefix search efficiency, insertion/deletion scalability, without depending on a particular data structure implementation.
This high level representation isolates the key functionality we need from a string storage and manipulation structure. Let me know if any aspects could be characterized more abstractly!
Terminology
Here are some key technical concepts relevant to understanding this trie problem:
Prefix tree  A tree data structure where strings are stored based on their prefixes to optimize searching and space efficiency. Critical because it underpins the problem.
Node  A basic unit in the tree storing data like a character. Nodes make up the trie.
Edge  Connections between nodes according to the prefix relationships. Edges form the tree structure.
Alphabet  The set of allowable characters for strings, which determines connectivity in the trie. Here it is just English lowercase letters.
Degree  Number of edges coming from a node. Limits node branching factor.
Path  Sequence of nodes traversed from root to a leaf. Paths represent inserted strings.
Key Concepts:
Prefix sharing  Storing common prefixes once. Crucial for compactness.
Lookup efficiency  Rapid searches based on shared prefixes. Core objective.
Insertion algorithm  Methodically adding nodes/edges for new strings. Need to understand this.
Prefix search  Leveraging structure to quickly count keys with a common prefix. Key operation.
In summary, technical concepts like prefix trees, nodes/edges, degree, paths, and prefix sharing underpin how tries achieves efficient search and storage. Understanding these concepts is vital for solving the problem optimally.
Problem Simplification and Explanation
Let’s break this down into simpler concepts and an analogy:
Key concepts:
Strings  The basic data items we need to store, like words in a dictionary
Prefixes  Initial letter chunks that are shared between strings, like “un” in “understand”
Tree structure  Strings get stored in a tree based on shared prefixes
Operations  Mainly inserting strings, finding strings, counting prefix matches, deleting strings
Efficiency  Doing the operations quickly, especially shared prefix searches
Analogy:
Think of the strings of letters as paths through a forest, where each node is a intersection. Strings that start the same branch into the same first intersections.
Inserting a string is like adding a new path through the forest intersections.
Counting strings with a prefix is like standing at the start intersection and counting paths that begin the same way.
Deleting a string is like blocking off a path through the intersections.
So in simpler terms, the key ideas are: store strings efficiently in a tree using prefix similarities to enable fast insertion, searching, and deletion operations on the strings. And we can analogize this to navigating a forest path network. Let me know if any part needs more clarification!
Constraints
Based on the problem statement, here are some specific characteristics we can leverage:
Strings are lowercase English letters only  This limited alphabet means trie node degree is at most 26, a small fixed constant. Can optimize storage and traversal.
Strings are relatively short  Max length 2,000 chars means we won’t need to optimize for extreme lengths. Traversal stack won’t get too large.
Many shared prefixes likely  For English text, many words share common prefixes and suffixes. This redundancy means we can heavily optimize for prefix factorization.
Count operation specified  Since counting shared prefixes is a key operation, we can optimize the node structure and algorithms specifically for fast traversal and counting.
Insert/delete costs amortized  At most 30K operations total. With average case fast. So we can take some liberty in instance complexity.
Balance not required  No need to keep optimal balance since worst case doesn’t dominate with amortized analysis over the operations.
The restricted string lengths, alphabet size, and ability to amortize costs all mean we can tune the trie nodes, pointers, and algorithms specifically for fast prefix counting without going overboard optimizing for extreme cases unlikely to occur in practice. The characteristics encourage simplicity without heavy tradeoffs.
Analyzing the constraints provided in the problem statement gives us a few key insights:
Small fixed alphabet  With only lowercase English letters, we know nodes have limited degree and we can optimize storage and traversals. We don’t need complex variablewidth encoding.
Short strings  The maximum length of 2000 allows us to optimize for “average case” of typical strings vs extreme worst case lengths. Storage and traversals can be simpler.
Prefix sharing  We can take advantage of significant prefix overlap between words/strings to heavily optimize search, storage, and traversal based on shared prefixes. This is a big opportunity.
Count operation focus  Since counting prefixes is a key operation, we can streamline the node structure and algorithms specifically for fast counting of matching prefixes.
Amortized costs  The limit of 30K operations means we can amortize work over time. We don’t need to obsess over any single operation’s complexity.
Balance not needed  Lack of balance requirements and amortized operations means we can simplify structure for faster traversal vs perfectly balanced trees.
The key takeaways are the ability to leverage fixed narrow alphabets, modest string lengths, high prefix overlap, and amortized costs to simplify storage, optimize for specific operations like counting, and generally streamline both data structures and algorithms  avoiding overengineering.
Case Analysis
Here are some additional test cases covering a wide range of inputs:
 Empty trie
Input: [“Trie”, “insert”, “countWordsEqualTo”, “countWordsStartingWith”] [[], [], [“apple”], [“app”]]
Output: [null, null, 0, 0]
This covers an empty trie. All counts will be 0. Useful to test initialization.
 Single string trie
Input:
[“Trie”, “insert”, “countWordsEqualTo”, “erase”, “countWordsEqualTo”]
[[], [“apple”], [“apple”], [“apple”], [“apple”]]
Output: [null, null, 1, null, 0]
Tests singleton string behavior. Insert one string and validate counts before and after deleting it.
 Repeated string
Input: [“Trie”, “insert”, “insert”, “countWordsEqualTo”] [[], [“test”], [“test”], [“test”]]
Output: [null, null, null, 2]
Useful for testing handling duplicates. Inserting a string multiple times should increase the count each time.
 Prefixes
Input: [“Trie”, “insert”, “countWordsStartingWith”] [[], [“app”, “apple”, “application”], [“app”]]
Output: [null, null, 3]
Demonstrates shared prefix counting. Inserting strings with a common prefix then counting that prefix.
 Mixed casing
Input: [“Trie”, “insert”, “countWordsEqualTo”] [[], [“Apple”], [“apple”]]
Output: [null, null, 0]
Tests case sensitivity. Inserting same word with different casing should not match.
Edge Cases:
 Empty strings
 Extremely long strings
 NonEnglish characters
 Empty trie operations
The key is covering a diverse set of string patterns  unique, repeated, overlaps, mixed casing, varying lengths  on both empty and populated tries to validate corner cases and robustness.
Here are some ideas for visualizing these test cases:
Draw the trie structure for each test case to show the state of the tree. This illustrates insertion, removal, and counting logically.
Use stepbystep animations to show the progression of insertions and deletions and how the trie changes over time for each operation.
Color code or visually highlight nodes and edges being accessed during traversal to visualize counting prefixes.
Use size or shading of nodes to represent counts at each node. Larger, darker nodes have higher counts.
Animate the traversal path for prefix queries by traversing the tree visually.
Show memory usage, balancing, and node size differences between test cases using tree visualization.
Depict a table showing string inputs and expected outputs for each test case. Cross these off as tests pass.
For edge cases, draw contrasting examples like extremely unbalanced tries to highlight difference from typical.
Use real text corpora to generate more realistic test data and tries.
The goal is to leverage visuals like animations, diagrams, tables, colors, etc. to fully illustrate the workings of each test case and amplify understanding of how the trie handles different conditions. Visualization makes abstract logical operations tangible.
Analyzing these test cases provides a few key insights:
Need to handle empty trie edge case properly, not just initialized structure
Singleton strings and counts are simple cases but essential to validate
Repeated insertions must increment count correctly
Prefixes require traversing and counting partial paths not just full strings
Case insensitivity is not intrinsic  we must handle case folding
Need to test a variety of string lengths from small to large
Mix of unique, duplicate, and prefix strings should be covered
Deletions should be tested on both populated and empty tries
Structure should be visually inspected for balance, height, node size
Memory usage impacts should be checked for scalability
Real text examples provide more robustness against unpredictable data
The main takeaways are the need to validate correctness and robustness on a diverse set of string patterns, operations, data structures, memory use, and algorithm behaviors using visual inspection and realworld data to supplement logical tests. Broad test coverage is key.
Identification of Applicable Theoretical Concepts
Some mathematical and algorithmic concepts that could help manage this problem:
Hash functions  Could hash strings to map to trie nodes, providing direct access and potentially optimizing search over linear traversal.
Bitwise operations  Bit manipulations could optimize compact storage of variable length strings in nodes.
Information theory  Prefix encoding concepts like Huffman coding optimize storage based on frequency.
Bloom filters  Could quickly rule out prefix absence before full traversal. Reduces average traversal length.
Finite state automata  The trie is essentially a state machine, transitioning between nodes. State machine theory provides optimization insights.
Discrete math  Prefix trees use combinatorics (combinations of strings). Graph theory concepts analyze complexity.
Amortized analysis  Amortize costs over sequences of operations to streamline individual operation complexity.
Probabilistic analysis  Model likelihood of prefixes based on statistical properties of strings to optimize expected performance.
In summary, concepts from hashing, bit logic, automata, information theory, discrete math, and algorithm analysis can provide theoretical grounding to develop, refine, and optimize trie data structures and algorithms. Mathematical abstractions help tackle concrete implementation challenges.
Simple Explanation
Here’s how I would explain this problem in simple nontechnical terms:
Let’s imagine you have a big dictionary of words. Now you want to store this dictionary nicely in a computer program so that you can easily:
 Add new words
 Count how many times a word appears
 Count how many words start with the same few letters
 Remove words
We want to store the dictionary efficiently, without wasting space repeating the same letters over and over. Like if the words “cat”, “cats”, “catch” are in the dictionary, we just want to store “cat” once and remember that other words start with those 3 letters.
This data structure works kind of like branches on a tree. Each branch is part of a word. Words that start the same branch off from the trunk. We use this tree shape so we can quickly count words sharing the same first few branch turns.
It’s like that game where you choose left or right paths through a maze. Except now imagine many people building word paths in the maze. We could quickly count how many paths start left, right, left by just looking at the first turns.
So in simple terms, we want to store a dictionary efficiently in a branched structure so we can easily find, count, add, and remove words. The shared branches let us search parts of words quickly.
Problem Breakdown and Solution Methodology
Here is how I would approach solving this trie problem in detail:
 Define Node structure
Each node will store:
 A character
 Links to child nodes (letters that can follow it)
 A count of terminal strings below it
This allows compact storage factoring out common prefixes.
 Implement insertion
To insert a string, start at root, add nodes for each letter that don’t exist, incrementing child links and counts.
E.g. inserting “cat”, add nodes for ‘c’, ‘a’, ’t’ with corresponding links.
 Implement searching
To count strings with a prefix, traverse nodes matching prefix letters. Sum count values along the path.
E.g. to count strings starting with “ca”, traverse path for ‘c’, ‘a’, summing counts.
 Implement deletion
Delete string by traversing its path, decrementing node counts, and removing child links and nodes if counts become 0.
E.g. delete “cat” by descending to leaf, decrementing counts, removing empty ’t’ node.
 Complexity analysis
Insertion and deletion are O(L) where L is string length. Searching for a prefix is O(P) where P is prefix length.
So we achieve optimizations for compact storage and fast prefix search.
Example case:
Insert “cat”, “cats”, “sat”
Search for strings starting with “ca” > Returns 2 because “cat”, “cats” match.
Delete “cats”
Search again > Now only “cat” matches so returns 1.
This shows inserting strings with shared prefixes, leveraging the structure to quickly search by prefix, and correctly maintaining counts when deleting strings.
Inference of ProblemSolving Approach from the Problem Statement
Here are key terms and how they guide the approach:
Trie  Indicates using a prefix tree structure to store strings compactly based on shared prefixes. This guides the node format and linkages.
Prefixes  Prefix searches being a key operation implies optimizing the data structure and algorithms for fast traversal based on shared initial letters.
Insert  We need to efficiently insert new strings by traversing and adding new nodes and edges. This drives the insertion logic.
Search  Supporting fast prefix searches guides creating efficient traversal algorithms and leveraging node counts.
Delete  Needing to remove strings indicates requiring deletion logic traversing edges and resetting counts.
Count  Optimizing count storage, updating, summation, and reporting should be prioritized since counting prefixes is a core operation.
Strings  Storing text strings guides choices like case sensitivity, allowing only letters, and optimizing for relatively short English words.
Efficiency  The need for speed on searches and acceptable insert/delete costs guides overall optimization tradeoffs.
So in summary, the core concepts of tries, prefixes, insertion/deletion/searching, counts, and efficiency considerations directly inform key decisions like data layout, algorithm approaches, and complexity optimizations. The terms shape the solution.
We can use tables and diagrams to visualize some of the key properties and aspects of this trie problem:
Trie structure  Draw the trie nodes and edges representing a sample of inserted strings. Illustrates compact prefix factoring.
String storage  Show strings as paths through the tree from root to leaves. Highlights branching by common prefixes.
Node format  Table columns showing character stored, child links, and count in each node. Communicates node structure.
Insertion logic  Stepbystep diagram showing new nodes added during an insertion. Conveys process flow.
Search traversal  Annotate trie with highlighting or annotations to depict traversing a prefix search.
Deletion  Diagram showing deletion of a node and resetting child links and counts. Visualizes removal process.
Memory  Tree diagram with node sizes or table with memory consumed by each node. Communicates memory use.
Time complexity  Table relating operations like search and insertion to big O complexity. Summarizes efficiency.
The goal is to leverage visual artifacts like trees, tables and annotations to illustrate the logical workings, data structures, algorithms, efficiency, and other key properties central to understanding the problem and solution.
Stepwise Refinement
Here is one way we could break this down into more refined, granular steps:
Highlevel approach:
 Implement a trie data structure to store strings
 Write methods to insert, delete, search, and count strings
Further refined steps:
Define trie node structure
 Character field
 Link array to child nodes
 Count field
Implement node insertion method
 Follow/add links matching string characters
 Increment count at terminus
Implement string deletion
 Traverse path to delete string
 Decrement counts and remove nodes
Implement prefix search
 Traverse nodes matching prefix
 Accumulate counts along path
Implement whole string search
 Invoke prefix search for full string
Some independent subproblems are:
 Node definition
 Insertion and deletion logic
 Prefix vs whole string search
Repeatable patterns include:
 Tree traversal logic
 Manipulating counts at nodes
 Sharing search prefix logic between operations
By iteratively refining the overall approach into smaller modular steps, we can tackle the subproblems independently and identify repetitive logic to simplify the implementation.
Solution Approach and Analysis
Here is how I would approach solving this trie problem in detail:
 Define Node structure
Each node will store a character, links to child nodes, and a count. This allows compactly storing strings as paths, factoring out shared prefixes.
 Implement insertion
To insert a string, start at root, traverse or add nodes for each letter, incrementing child links and count at end. This builds out the trie to store the new string.
 Implement searching
To count strings with a prefix, traverse nodes matching prefix letters, summing counts along the path. This leverages the compact structure.
 Implement deletion
Delete a string by traversing its path in the trie, decrementing node counts, removing child links and nodes if counts become 0. This maintains the trie contents.
 Complexity analysis
Insertion and deletion take O(L) time where L is string length. Searching takes O(P) time where P is prefix length. This optimizes prefix searches.
Example walkthrough:
Insert “cat”, “dog”, “catalog” Search for strings starting with “cat” Returns 2 because “cat”, “catalog” match Delete “catalog” Search again, now only “cat” matches So returns 1
This shows inserting distinct strings with some overlap, leveraging the trie structure to efficiently search prefixes, and correctly updating the node counts when deleting entries.
The overall process is:
 Model the data structure
 Implement insertion
 Implement deletion
 Implement search
 Analyze time complexities
And verify with example cases. The key is the layered, stepbystep methodology. Let me know if any part needs more clarification!
Identify Invariant
The invariant in this trie problem is:
At any point during insertion, deletion or searching, the traversal path from the root to the current node represents a valid prefix of strings stored within the subtrie rooted at that node.
In other words, the sequence of characters obtained by traversing from the root to any node is guaranteed to be a prefix shared by all strings reachable from that node.
This invariant enables efficient prefixbased searches, insertions, and deletions by ensuring the trie structure always preserves the prefix relationship as strings are added and removed.
Without this invariant, the trie could become an arbitrary tree and would no longer support efficient prefix queries. But by maintaining this prefix property, the trie can directly implement operations like:
 Find all strings with a given prefix by traversing to the prefix’s node
 Insert a string by traversing or creating nodes for each prefix
 Delete a string by traversing its path then pruning branches
So this prefixpreserving structure provides the fundamental trie functionality. The methods rely on traversing known prefixes indicated by the current node path.
Identify Loop Invariant
The key loop invariant in operations like search and traversal on the trie is:
At each step in the traversal, the path traversed so far from the root is a prefix of the target string.
More formally:
Let P be the path traversed so far, S be the target string, and i be the current step.
Loop Invariant: P is a prefix of S after i iterations.
This holds because:
 Before the loop, P and i are initialized properly.
 Each iteration extends P by one node matching the next character in S.
 So P can only contain prefixes of S.
This invariant is used in:
 Searching  Traversing matching prefix nodes ensures we follow a prefix path.
 Insertion  Traversing/adding prefix nodes maintains prefix tree structure.
 Deletion  Traversing to target node ensures we find the string occurrence.
It enables sequentially traversing and manipulating the nodes along a string’s prefix path in the trie without deviating. The methods rely on this invariant to operate correctly.
Thought Process
Here is how I would approach solving this trie problem:
The main cues in the problem statement are:
 Manage a dataset of strings
 Support efficient operations like search, insert, delete
 Optimize for searching prefixes
This suggests using a trie (prefix tree) since it efficiently stores strings and enables fast prefixbased lookups.
The thought process involves these key steps:
Define the trie node structure to store characters, child links, and a count field. This will enable compact storage and fast traversal.
Implement insertion logic that descends through the trie, adding nodes for the string characters until reaching the end where the count is incremented.
Implement search by similarly traversing nodes matching the prefix, summing the count fields along the way. Return the total count.
Delete by traversing to the node, decrementing counts, then pruning any child branches where counts reached 0.
Optimize with techniques like keeping child links/nodes in arrays for direct access.
Here is some sample Python code sketch:


The key insights are to leverage the trie structure for factors like prefix sharing, apply recursive descent for insertion/deletion/search, and optimize operations like search using the counts.
Establishing Preconditions and Postconditions
Here is how I would analyze the insert
method for the trie:
Parameters:
word
 The string to insert Type is
String
 Represents the key we want to add to the trie
Preconditions:
 Trie is initialized and not
null
word
is not null or emptyword
only contains lowercase alphabet characters
Method Functionality:
 Descend tree matching
word
characters  Add nodes if needed
 Increment count at end node
 Modify tree structure to include
word
Postconditions:
word
is now present in the trie Count at terminal node is incremented
 Trie structure is updated with new nodes/links as needed
Error Handling:
 If
word
isnull
, throwNullPointerException
 If
word
is invalid (nonalphabet chars), throwIllegalArgumentException
 If any node creation fails, throw
OutOfMemoryError
Analyzing methods in this way helps define constraints, validate functionality, specify error handling, and clarify inputs, outputs and side effects  providing a precise specification.
Problem Decomposition
Here’s how I would break down this trie problem:
Problem Understanding: The goal is to implement a trie that stores strings compactly and allows efficient operations like insertion, deletion, and searches based on prefixes.
Initial Breakdown: The major parts are:
 Implementing the trie data structure
 Writing insertion logic
 Writing deletion logic
 Writing search logic
 Optimizing performance
 Subproblem Refinement: Key tasks under each part include:
 Defining the node format (character, children, counter)
 Traversing the trie when inserting/deleting
 Updating node links and counts
 Traversing prefixes during search
 Summing count fields along search path
 Pruning deletions
 Optimizing node storage
Task Identification: Common tasks include traversal, updating counts, pruning, node creation.
Task Abstraction: We can generalize the traversal logic by abstracting it into a
traverse
method. Updating counts can beincrementCount
anddecrementCount
.Method Naming:
insertWord
,deleteWord
,searchPrefix
,createNode
,traverse
,incrementCount
,decrementCount
Subproblem Interactions:
insertWord
relies ontraverse
andcreateNode
.deleteWord
usestraverse
anddecrementCount
.searchPrefix
leveragestraverse
.
So breaking the problem down in this structured way helps identify subtasks, generalize repeated logic, clarify interactions, and define a clean interface.
From Brute Force to Optimal Solution
Implementing a prefix tree, also known as a Trie, doesn’t typically involve a bruteforce solution because the data structure is specifically designed for efficient operations like insertion, deletion, and search. The idea of a Trie is to use a treelike structure where each node represents a single character of a string, optimizing the operations for which you might consider using bruteforce in a less specialized data structure.
In a Trie, insertion, search, and deletion generally have time complexity O(n), where n is the length of the word being inserted, searched, or deleted. These are already efficient operations, making a bruteforce approach unnecessary and less efficient.
A “bruteforce” approach to problems related to string search, for example, might involve scanning through every substring of every string to find a match. This would have a much worse time complexity compared to a Triebased solution.
So, while you could technically come up with a bruteforce approach for problems that a Trie can solve (like checking if a word exists in a set of words), the Trie itself is an example of a more efficient solution that makes bruteforce methods obsolete for those specific problems.
Here is one approach to optimizing a brute force solution for the trie problem:
Brute force solution:
 Store strings in an array
 Insert by adding string to array
 Search by looping through array, checking each string
 Delete by finding string and removing from array
This is inefficient because:
 No prefix factorization
 Searches are O(N) linear scans
 Inserts and deletes change array size
First optimization  Prefix tree:
 Store strings as paths in a tree
 Insert by adding nodes for string
 Search by traversing tree matching prefix
 Delete by traversing tree to string
This improves efficiency by:
 Prefix sharing reduces storage
 Searches become O(M) where M is prefix length
Next optimization  Node links:
 Make each node link directly to child nodes in array
 Insert by traversing node links
 Search by following links matching prefix
This further improves:
 Faster node access than traversing all children
Final optimization  Node counts:
 Add count field on each node
 Increment count when inserting
 Sum counts during search
This improves:
 Faster searching by aggregating counts in nodes
So optimizing from brute force array to a prefix tree to using node links and counts improved time complexity from O(N) to O(M) and reduced storage by sharing prefixes. The stepbystep process illuminates how each change incrementally improves efficiency.
Code Explanation and Design Decisions
Here’s how I would analyze the logic for searching for a prefix in the trie:
Initial parameters are the
root node
of the trie and theprefix
string to search for.The main loop traverses down the trie, starting from the root, following links matching the prefix characters. Each iteration represents traversing one node in the prefix path.
Branches check if the current character of the prefix matches the node value. This ensures we follow the path corresponding to the prefix.
A running
count
variable sums the count field of each node visited. This aggregates the number of strings under that prefix as we traverse down.Loop invariant is that the path traversed so far is a prefix of the search prefix. This maintains alignment with the target prefix being searched.
Final output is the total count under the prefix path. This satisfies the goal of counting number of strings matching the prefix efficiently.
In summary, the logic descended into the trie matching the target prefix, aggregating counts along the way to efficiently compute strings matching the searched prefix while ensuring correct path traversal.
Coding Constructs
Here is how I would analyze the highlevel logic of this trie prefix search code:
This code uses tree traversal as the core strategy, leveraging the trie’s prefix tree structure to efficiently find strings.
At a high level, it’s traversing a tree of letters where common starting letters share branches, counting strings stored under those shared prefixes.
The key logical constructs are recursion/iteration to traverse the tree, conditional branches to match letter by letter, and aggregation by summing node counts.
The algorithm follows links matching the prefix letters, at each step summing the count of strings at that node, recursively descending the tree until the prefix is matched.
The steps are: initialize counter, compare prefix letter, follow child link, add node count to counter, recursively descend until prefix fully matched. This leverages the prefix tree structure.
It employs recursive tree traversal, conditional iteration with backtracking, aggregation of intermediate results, and early termination when goal reached.
In summary, the core of the logic involves recursively traversing the trie matching prefix letters, aggregating node counts to tally strings under that prefix, leveraging the inherent tree structure. The code encapsulates arboreal search and aggregation patterns.
Language Agnostic Coding Drills
Here’s how I would break down implementing a trie into fundamental coding concepts:
Basic Concepts:
 Variables to store data values
 Conditionals (ifstatements)
 Loops for iteration
 Functions for modularity
 Hash table/map for fast lookup
Intermediate Concepts:
 Recursive functions
 Dynamic arrays/linked lists
 Tree traversal (DFS/BFS)
 Search algorithms like binary search
Advanced Concepts:
 Optimal binary search trees
 Space/time complexity analysis
 Memory management/pointer manipulation
 Multiway tree implementation
The key is to first establish basic language constructs, then build up to generalized patterns like recursion and search algorithms, finally optimizing with advanced analysis and custom trees.
The problemsolving flow would involve:
 Implement core language features like arrays, lists, conditionals
 Build general tree traversal algorithms
 Apply tree search to implement trie insertion and lookup
 Optimize with advanced analysis and custom nodes
Each coding concept provides a modular building block that can be combined to construct an efficient customized trie stepbystep.
Targeted Drills in Python
Here are some example Python coding drills for key concepts needed to implement a trie:
Basic Python:


Intermediate concepts:


Problemspecific:


These can be combined to:
 Implement language fundamentals
 Build up recursive algorithms and data structures
 Apply tree logic to customize trie nodes and operations
The key is breaking the problem down into modular coding skills that are assembled like Lego blocks into the final solution.
Q&A
Similar Problems
Here are 10 problems that involve similar concepts to implementing a trie prefix tree:
Design Add and Search Words Data Structure (208)  Also implements add/search operations on a trie prefix tree structure.
Replace Words (648)  Manipulates strings by traversing a trie while matching prefixes.
Maximum XOR of Two Numbers in an Array (421)  Uses a trie to efficiently compute XORs based on shared prefixes.
Sort Characters By Frequency (451)  Counts character frequencies then manipulates strings accordingly, similar to using trie node counts.
Number of Distinct Substrings in a String (3187)  Involves constructing a trie and querying prefixes, similar pattern.
Design Autocomplete System (642)  Autocomplete is a common application of tries, shares prefix search logic.
Stream of Characters (1032)  Processes streams of text by building up a trie incrementally.
Concatenated Words (472)  Set of problems around constructing and querying string collections, like our trie operations.
Short Encoding of Words (820)  Encoding strings by shared prefixes relates to how tries store strings.
Delete Leaves with a Given Value (1325)  Requires recursively traversing and deleting trie nodes similar to our delete operation.
The shared elements are string manipulation using variants of trie trees and prefixes along with operations like insert, delete, search and counting frequency. These related problems leverage similar data structures and algorithms.