Hackernoon how to implement trie (prefix tree) – blind 75 leetcode questions

hackernoon how to implement trie (prefix tree) - blind 75 leetcode questions

In the realm of competitive programming and technical interviews, the ability to efficiently implement data structures and algorithms is paramount. One such fundamental data structure that proves invaluable in various scenarios is the Trie, also known as the Prefix Tree. In this comprehensive guide, we will explore the intricacies of implementing a Trie and delve into its applications, specifically focusing on solving Blind 75 LeetCode questions. Let’s embark on a journey to understand the Trie and enhance our problem-solving skills.

Understanding Trie and Its Significance

What is a Trie?

A Trie, or Prefix Tree, is a tree-like data structure that is used for efficient storage and retrieval of a dynamic set or associative array. Unlike traditional tree structures, such as binary trees, Tries are particularly useful for scenarios involving strings and sequences. The name “Trie” is derived from the word “retrieval,” and it excels at facilitating fast prefix-based searches.

Why Trie?

Tries offer several advantages, especially when dealing with problems that involve words or sequences. Some key benefits include:

Fast retrieval

Tries provide quick and efficient access to stored data, making them ideal for scenarios where fast search operations are crucial.

Prefix matching

Tries are specifically designed for handling prefix-based searches, making them suitable for autocomplete systems and word dictionaries.

Space efficiency

Tries are memory-efficient for storing large datasets with common prefixes, reducing redundant storage.

Implementing a Trie

Step-by-Step Guide

Node Structure

The foundation of a Trie lies in its nodes. Each node represents a character in a word, and the edges between nodes denote the connection between characters. Let’s define the structure of a Trie node in a programming language of your choice (e.g., Python).

class TrieNode:
def __init__(self):
self.children = {}
self.is_end_of_word = False

Here, children is a dictionary that maps characters to their corresponding Trie nodes, and is_end_of_word indicates whether the Insertion Operation

The insertion operation involves adding a word to the Trie. The process is recursive and straightforward.

def insert_word(root, word):
node = root
for char in word:
if char not in node.children:
node.children[char] = TrieNode()
node = node.children[char]
node.is_end_of_word = True

This function iterates through each character in the word, creating new nodes as needed, and marks the end of the word appropriately.

Search Operation

Searching for a word in the Trie is a matter of traversing the Trie and checking if each character exists.

def search_word(root, word):
node = root
for char in word:
if char not in node.children:
return False
node = node.children[char]
return node.is_end_of_word

This function returns True if the entire word is found in the Trie and False otherwise.

Trie Implementation

Now, let’s tie everything together and implement a Trie class that encapsulates these operations.

class Trie:
def __init__(self):
self.root = TrieNode()
def insert(self, word):
insert_word(self.root, word)def search(self, word):
return search_word(self.root, word)

With this Trie implementation, we have a robust foundation for solving various problems, including the Blind 75 LeetCode questions.

Solving Blind 75 LeetCode Questions Using Trie

Now that we have a solid understanding of Trie implementation, let’s dive into solving Blind 75 LeetCode questions. These questions are carefully curated to cover a broad spectrum of topics, and Trie becomes a powerful tool for solving a subset of them efficiently.

Longest Word in Dictionary

Problem Statement

Given a list of strings words representing an English Dictionary, find the longest word in the dictionary that can be built one character at a time by other words in the dictionary.

Solution using Trie

We can use a Trie to store the words in a way that allows us to efficiently check if a word can be built character by character.

def longest_word(words):
trie = Trie()
for word in sorted(words):
if trie.search(word[:-1]):
return max(trie.root.children.keys())

This solution builds the Trie in a sorted order, ensuring that shorter words are inserted first. It then iterates through the words, checking if the prefix (all characters except the last one) is present in the Trie. If so, the word is inserted into the Trie. Finally, the longest word is determined by traversing the Trie to find the maximum key.

Word Search II

Problem Statement

Given a 2D board and a list of words from the dictionary, find all words in the board.

Solution using Trie

This problem involves searching for multiple words efficiently, making it an ideal candidate for a Trie-based solution.

def find_words(board, words):
trie = Trie()
for word in words:
def dfs(node, i, j, path, result):
if node.is_end_of_word:
node.is_end_of_word = False # Avoid duplicate resultsif 0 <= i < len(board) and 0 <= j < len(board[0]):
char = board[i][j]
child_node = node.children.get(char)if child_node:
board[i][j] = ‘#’ # Mark as visited
dfs(child_node, i + 1, j, path + char, result)
dfs(child_node, i – 1, j, path + char, result)
dfs(child_node, i, j + 1, path + char, result)
dfs(child_node, i, j – 1, path + char, result)
board[i][j] = char # Reset the board

result = []
for i in range(len(board)):
for j in range(len(board[0])):
dfs(trie.root, i, j, , result)

return result

This solution first builds a Trie from the given words. Then, it performs a depth-first search (DFS) on the board, checking if the current sequence forms a valid word in the Trie. The results are collected, and duplicate entries are avoided.


Mastering the implementation of a Trie (Prefix Tree) is a valuable skill for any programmer, especially when tackling complex algorithmic problems and technical interviews. In this article, we covered the fundamentals of Trie, step-by-step implementation, and applied our knowledge to solve Blind 75 LeetCode questions efficiently.

By understanding the Trie data structure and practicing its implementation, you are better equipped to approach a wide range of problems involving strings and sequences. The examples provided showcase how Tries can be utilized to solve real-world programming challenges, making them an essential addition to your algorithmic toolkit.

As you continue to explore and apply Trie-based solutions, remember to analyze the problem requirements, design your Trie structure accordingly, and leverage the power of Trie to optimize your code for faster and more efficient solutions. Happy coding!

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