Category: Python Coding Test

Introduction

Solving algorithm problems is a very important process for software developers. In particular, trees are used as a core structure in many problems, and understanding how to traverse trees is essential for effectively utilizing this structure. In this article, we will explain the basic concepts of trees and various traversal methods using Python, and connect theory to practice by solving actual algorithm problems.

Basics of Trees

A tree is a non-linear data structure consisting of nodes and the connections between them.
Each node can have child nodes, making this structure suitable for representing hierarchies.
The topmost node is called the root node, and there are various traversal methods that define the relationships between nodes.

Types of Trees

– Binary Tree: A tree where each node can have at most two child nodes.
– Binary Search Tree: A sorted binary tree where the left child is smaller than the parent node, and the right child is larger.
– AVL Tree: A balanced binary search tree that maintains a height difference of 1 or less.

Tree Traversal Methods

Tree traversal defines the order in which nodes are visited. The main traversal methods are as follows.

1. Pre-Order Traversal

Pre-order traversal visits the node itself first, then recursively visits the left child node, and finally visits the right child node.
In other words, the visiting order is: Node → Left → Right

2. In-Order Traversal

In-order traversal visits the left child node first, then the node, and finally the right child node.
The order is: Left → Node → Right

3. Post-Order Traversal

Post-order traversal visits the left child node first, then the right child node, and finally the node itself.
The order is: Left → Right → Node

Problem: Print the Results of Binary Tree Traversal

The following is a problem that takes nodes of a binary tree as input and outputs the results of pre-order, in-order, and post-order traversals.
Given a list containing the values of each node, you need to return the results of the tree traversal.

Problem Description

Based on the given list, construct a binary tree and return a list containing the results using each traversal method.
For example, if the list is [1, 2, 3, 4, 5], the following tree can be constructed:

                 1
                / \
               2   3
              / \
             4   5
            

Input / Output Format

• Input: A list containing each node.
• Output: Lists of results from pre-order, in-order, and post-order traversals.

Problem-Solving Process

Step 1: Define the Tree Structure

To construct the tree, we will first define a class that represents a node.

class TreeNode:
    def __init__(self, value):
        self.value = value
        self.left = None
        self.right = None
            

Step 2: Build the Tree

We will write a function that constructs the tree based on the given list.
In this example, we simply set the first value of the list as the root node and place the rest as the left and right children.

def build_tree(values):
    if not values:
        return None
    root = TreeNode(values[0])
    for value in values[1:]:
        insert_node(root, value)
    return root

def insert_node(root, value):
    if value < root.value:
        if root.left is None:
            root.left = TreeNode(value)
        else:
            insert_node(root.left, value)
    else:
        if root.right is None:
            root.right = TreeNode(value)
        else:
            insert_node(root.right, value)
            

Step 3: Write the Traversal Functions

We will implement each traversal method. The tree will be explored recursively and nodes will be stored in a list.

def preorder_traversal(root):
    result = []
    if root:
        result.append(root.value)
        result.extend(preorder_traversal(root.left))
        result.extend(preorder_traversal(root.right))
    return result

def inorder_traversal(root):
    result = []
    if root:
        result.extend(inorder_traversal(root.left))
        result.append(root.value)
        result.extend(inorder_traversal(root.right))
    return result

def postorder_traversal(root):
    result = []
    if root:
        result.extend(postorder_traversal(root.left))
        result.extend(postorder_traversal(root.right))
        result.append(root.value)
    return result
            

Step 4: Integrate and Call

Finally, we integrate all of the above functions to solve the problem.

def traverse_tree(values):
    root = build_tree(values)
    return {
        'preorder': preorder_traversal(root),
        'inorder': inorder_traversal(root),
        'postorder': postorder_traversal(root),
    }

# Example usage
input_values = [1, 2, 3, 4, 5]
result = traverse_tree(input_values)
print(result)  # Print the results
            

Final Results and Summary

Through the above process, we can traverse the given binary tree list and derive the results of pre-order, in-order, and post-order traversals.
This problem has taught us the concept of tree traversal and how to implement it efficiently.
Additionally, we encourage you to experience and practice with various tree structures and traversal methods.

1. Introduction

Hello! Today, we will explore the very useful Trie data structure for job seekers. To solve various algorithm problems, one must understand and utilize different data structures, among which the Trie data structure is very powerful in effectively solving string-related problems. In this post, we will cover the concept, characteristics, examples of use, and the problems that can be solved using Tries.

2. What is the Trie Data Structure?

A Trie is a tree structure optimized for storing multiple strings. It is primarily used for prefix searches. Each node typically represents one character of a string, and by starting from the root node and progressing down through child nodes, one can interpret each character of the string step by step. A Trie has the following characteristics:

• Each node in the Trie has characters and its child nodes.
• Strings are constructed through the path of the nodes.
• Duplicate strings are merged into a single path.
• Each node can mark the end of a string to indicate its termination.

3. Trie Structure

The Trie structure can be designed as follows. Each node corresponds to a single character, and as one descends from the root node, strings are formed. For example, adding the strings ‘cat’ and ‘car’ would create the following structure:

        Root
        └── c
            ├── a
            │   ├── t (end node)
            │   └── r (end node)
        

This structure allows for easy exploration of strings with the prefix ‘ca’.

4. Implementing a Trie

To implement a Trie, we will start by defining classes and methods. The basic implementation involves defining nodes and creating a Trie class with functionalities for insertion, search, and deletion.

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

class Trie:
    def __init__(self):
        self.root = TrieNode()

    def insert(self, word: str) -> None:
        node = self.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

    def search(self, word: str) -> bool:
        node = self.root
        for char in word:
            if char not in node.children:
                return False
            node = node.children[char]
        return node.is_end_of_word

    def starts_with(self, prefix: str) -> bool:
        node = self.root
        for char in prefix:
            if char not in node.children:
                return False
            node = node.children[char]
        return True
        

The above code defines the TrieNode class that represents a Trie node and the Trie class that includes basic Trie functionalities.

5. Problem: Finding Prefixes in a List of Strings

Now, let’s use the Trie to solve the ‘prefix search’ problem. The problem is as follows.

Problem Description

Given a list of strings, write a program to verify if a specific string is a prefix of any string in this list.

Input

- List of strings: ["apple", "banana", "app", "apricot"]
- Prefix: "app" 

Output

'app' is a prefix of 'apple' and 'app' in the list.

Now let’s use a Trie to solve this problem.

6. Problem Solving Process

We will use the Trie to solve this problem. First, we will insert the list of strings into the Trie, then check if the given prefix is included in the Trie.

def find_prefix_words(words: List[str], prefix: str) -> List[str]:
    trie = Trie()
    for word in words:
        trie.insert(word)

    result = []
    for word in words:
        if word.startswith(prefix):
            result.append(word)
    return result

# Example usage
words = ["apple", "banana", "app", "apricot"]
prefix = "app"
print(find_prefix_words(words, prefix))
        

The code above implements a function that takes a list of strings and finds the prefixes of that list. It is designed to return all strings that start with the user-provided prefix.

7. Conclusion

Tries offer significant performance improvements in string data processing and searching. They are a very powerful tool for solving problems like prefix searches. As discussed in this post, using Tries allows for the effective handling of character-based data. I hope today’s post has helped you understand the Trie data structure.

Look forward to more posts covering various algorithm problems!