20.3.6 - Complexity Analysis of BFS
Enroll to start learning
You’ve not yet enrolled in this course. Please enroll for free to listen to audio lessons, classroom podcasts and take practice test.
Interactive Audio Lesson
Listen to a student-teacher conversation explaining the topic in a relatable way.
Graph Representation
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Today we are going to discuss how graphs can be represented and how these representations affect the complexity of BFS. Can anyone tell me how we might represent a graph?
We can use an adjacency matrix, right?
Correct! An adjacency matrix is a 2D array where each cell indicates if there is an edge between two vertices. What is the downside of using an adjacency matrix?
It can take a lot of space, especially if the graph is sparse.
Exactly! That's why we often use an adjacency list instead, which only records existing edges. This can greatly improve efficiency. Remember, an easy way to think about this is: less density, less space!
So BFS would run faster with an adjacency list?
Yes! In terms of complexity, using an adjacency list, BFS runs in O(n + m) time compared to O(n²) with an adjacency matrix. Let's remember: 'A List for Speed!'.
Complexity Analysis of BFS
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Great, now let's dive deeper into BFS's complexity. If we look at an adjacency matrix, how many times would a vertex be processed?
Once for each vertex?
Correct! Each vertex enters the queue once, but to explore its neighbors, we might look at every other vertex too. This leads us to O(n²). However, with an adjacency list, what happens?
We only check the neighbors, leading to O(m) for edges!
Exactly! We can summarize this as: 'Matrix Time is Quadratic, List Time is Linear'.
So for sparse graphs, lists are better!
You got it! Remember, the efficiency with which we explore the graph is key to optimal performance!
Path Reconstruction in BFS
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Now, let's talk about path reconstruction. When BFS visits a vertex, how can we keep track of how we got there?
We can use a parent array to store where each vertex came from.
Exactly! By recording the parent of each visited vertex, we can backtrack from any node to the source. This makes BFS great for finding the shortest path in unweighted graphs. Can anyone think of a real-world example?
Finding the shortest route on a map!
Exactly, GPS navigation can leverage this concept. Our mantra here can be: 'Trace Back the Path, Find Your Way!'
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
In this section, we explore the actual algorithm and its complexity when using both an adjacency matrix and adjacency list representation. It demonstrates how BFS performs optimally in terms of graph structure and edges, highlighting the difference in time complexity between the two methods.
Detailed
Complexity Analysis of BFS
In this section, we analyze the complexity of the Breadth-First Search (BFS) algorithm in graph exploration. The BFS algorithm systematically explores vertices level by level, starting from a source vertex and using data structures to track visited vertices and queue unexplored ones.
Graph Representation
The section discusses representing graphs using:
1. Adjacency Matrix: A 2D array where matrix[i][j] indicates an edge between vertices i and j. Although efficient for small graphs, its time complexity becomes O(n^2) due to the need to scan rows of the matrix for neighbors.
2. Adjacency List: A more space-efficient representation ideal for sparse graphs, where only existing edges are recorded. This allows exploring neighbors in linear time relative to edges, leading to a combined complexity of O(n + m), where n is the number of vertices and m is the number of edges.
Complexity Discussion
The complexity analysis emphasizes that in the adjacency matrix representation, each vertex is processed once with a cost of examining all possible edges, hence O(n^2) complexity if the graph is dense. Conversely, with an adjacency list, each edge is only explored a limited number of times, ensuring it runs in linear time, which is critical for efficiency.
Path Reconstruction
Moreover, BFS can efficiently store paths and levels for vertices during exploration, contributing to functionalities like finding the shortest path in unweighted graphs. Each visited node can maintain a reference to its parent, facilitating path reconstruction from any node back to the source.
In summary, understanding the complexity of BFS helps in choosing appropriate representations and optimally executing graph traversal algorithms.
Youtube Videos
Audio Book
Dive deep into the subject with an immersive audiobook experience.
Introduction to Complexity Analysis
Chapter 1 of 6
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
How do we analyze the complexity of an algorithm? One way to do it is to look at the loop. So, remember that, when we have an algorithm like this, which has the loop, the loop is usually the place where we have to look for it.
Detailed Explanation
Analyzing the complexity of an algorithm often starts by examining its loops. Loops are crucial because they determine the number of times certain operations are repeated. For instance, in Breadth First Search (BFS), the loops will reveal how many vertices are processed and how connections between them are explored.
Examples & Analogies
Think of analyzing an algorithm like counting how many times you go up and down the stairs in a building. Each trip up or down (like each loop iteration) contributes to the total effort, and understanding how many trips there are helps in knowing how much energy you will spend.
Outer Loop Execution
Chapter 2 of 6
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
Each vertex enters the queue exactly once. So, it means that this outer loop will take order n time. Because, assuming the graph is connected, every vertex will be visited once...
Detailed Explanation
In BFS, each vertex is added to the queue only once. Therefore, when counting the total operations in the outer loop, we notice that it runs in linear time, denoted as O(n), where n is the number of vertices. This efficiency is significant because it means that as the size of the graph grows, the number of iterations of this outer loop increases linearly.
Examples & Analogies
Imagine you are sending invitations to a wedding. If you have 100 guests, you have to send 100 invitations—that's a direct relationship. However, if you had 10,000 guests, you would have 10,000 invitations, still following a straightforward pattern without any unnecessary extra steps.
Examining Neighbors
Chapter 3 of 6
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
For each vertex, we have to scan all its neighbors, and this will be O(n) if we use an adjacency matrix.
Detailed Explanation
When a vertex in BFS is processed, the algorithm examines its neighbors to see which ones haven't been visited yet. If an adjacency matrix is used to represent the graph, this examination involves scanning a full row corresponding to the vertex, which has a time complexity of O(n). This means that for every vertex processed, potentially every other vertex needs to be checked for connections.
Examples & Analogies
Consider a social network where you are checking a friend's friends. If each friend has a full list of connections (like an adjacency matrix), you would have to check every name in that list to find connections, which can be time-consuming if there are many connections.
Overall Complexity Understanding
Chapter 4 of 6
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
So, therefore, if the graph is connected, every vertex will get into the queue. So, we will do the queue loop exactly n times. And for each j that is extracted, we need to examine all its neighbors...
Detailed Explanation
Putting together the knowledge of the outer loop and the neighbor examination, we can say that the overall complexity of the BFS algorithm is O(n²) when using an adjacency matrix. This arises because each vertex is processed once, leading to n iterations, and for each of those iterations, we potentially check connections for each vertex, leading to an additional n complexity.
Examples & Analogies
Think of a library where each book (vertex) has a full list of all other books it references (connections). If you check each book to see its references and repeat this process for every book, you will spend a considerable amount of time looking through each reference, much like the O(n²) complexity reflects.
Adjacency List Representation
Chapter 5 of 6
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
If we have very few edges, we can improve the complexity by using an adjacency list representation instead of the adjacency matrix.
Detailed Explanation
Switching from an adjacency matrix to an adjacency list helps reduce the number of checks needed. In an adjacency list, each vertex holds only the neighbors it connects to, so when checking for connections, we only look at a small number of relevant vertices, leading to a time complexity of O(n + m), where m is the number of edges. This makes BFS significantly faster when sparsely connected.
Examples & Analogies
Imagine you are at a party with very few guests (edges) and you have a guestbook that only lists people you know. You quickly scan through just your circle, rather than sorting through a huge list of everyone at the party. This makes finding connections much faster and simpler.
Path Reconstruction and Levels
Chapter 6 of 6
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
To reconstruct the path, we can remember where we marked each vertex from, thus allowing tracing back the path found...
Detailed Explanation
In BFS, we can identify not only which vertices are connected but also how we reached them by keeping track of a 'parent' for each vertex. This parent information allows us to retrace our steps from any vertex back to the source. Additionally, by recording the levels of each vertex explored—based on how many edges away they are from the source—we understand the distance to each vertex in terms of edges.
Examples & Analogies
Think of times when you’ve followed a route back home. If you kept a record of each turn (parent) you took and how far each was from your starting point (level), you'd easily be able to retrace your steps.
Key Concepts
-
Graph Representation: Different ways to represent graphs affect BFS performance.
-
Adjacency Matrix vs. List: The matrix leads to O(n²) complexity while the list leads to O(n + m).
-
Path Reconstruction: BFS can reconstruct paths by tracking parent nodes.
Examples & Applications
Using an adjacency matrix can be inefficient for large graphs while using a list optimizes the space and time.
When finding the shortest path in an unweighted graph, BFS will give the correct answer by exploring all nodes level-wise.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
BFS explores left and right, searching out all in sight!
Stories
Imagine a person named BFS who goes through a maze, taking turns at each section until they find their friend! In the process, they take note of all the paths taken to get back.
Memory Tools
Use the word 'SEARCH' to remind you of key BFS steps: Start, Explore, Add, Remove, Check neighbors, and Hop to the next level.
Acronyms
Remember the acronym 'MAPS' for graphs
Matrix is dense
Adjacency list is sparse
Paths can be rebuilt
Search is level by level.
Flash Cards
Glossary
- BFS
Breadth-First Search, an algorithm for traversing or searching graph data structures.
- Graph
A collection of vertices and edges connecting them.
- Adjacency Matrix
A 2D array that represents a graph where rows and columns indicate vertices.
- Adjacency List
A collection of lists where each list corresponds to a vertex and contains its adjacent vertices.
- Complexity
A measure of the resources required for an algorithm to complete, usually in terms of time and space.
- Path Reconstruction
The process of determining the sequence of vertices that lead from a source to a destination.
- Edge
A connection between two vertices in a graph.
Reference links
Supplementary resources to enhance your learning experience.