Common Graph Algorithms: Dijkstra's algorithm
Published: 2023-05-27 11:33:33

## Introduction
Graph theory provides powerful tools for solving complex problems, and one such tool is Dijkstra's algorithm. This algorithm efficiently finds the shortest path between two vertices in a weighted graph. In this blog post, we will explore the inner workings of Dijkstra's algorithm, discuss its time complexity, and showcase its real-world applications. Let's dive into the world of optimizing shortest paths with Dijkstra's algorithm.

## Understanding Dijkstra's Algorithm
Graph Representation: -
Before we delve into the algorithm, let's quickly recap how graphs are represented. A graph consists of vertices (also known as nodes) connected by edges. In Dijkstra's algorithm, we consider a weighted graph, where each edge has a non-negative weight representing the cost or distance between two vertices.

Key Concepts: -
- Priority Queue: We need a data structure that allows us to efficiently retrieve the vertex with the minimum distance during the algorithm's execution. This is typically implemented using a priority queue, which provides fast access to the minimum element.
- Visited and Unvisited Vertices: To keep track of the vertices that have been visited and the ones yet to be explored, we maintain two sets: the set of visited vertices and the set of unvisited vertices.

Step by step execution: -
- Initialization: We start by assigning a tentative distance value to every vertex. The distance of the source vertex is set to 0, and all other vertices are initialized with a value representing infinity. The source vertex is selected as the starting point.
- Main Loop: We iteratively select the vertex with the minimum distance from the set of unvisited vertices. Initially, this will be the source vertex. We then visit all the neighboring vertices of the selected vertex and update their tentative distances if a shorter path is found.
- Updating Tentative Distances: For each neighboring vertex, we calculate the distance from the source vertex through the current vertex. If this distance is shorter than the previously recorded distance, we update the tentative distance.
- Marking a Vertex as Visited: After visiting all the neighbors and updating their tentative distances, we mark the current vertex as visited and remove it from the unvisited set.
- Repeat: We repeat steps 2-4 until we have visited all the vertices or until the destination vertex is marked as visited.

Time Complexity analysis: -
The time complexity of Dijkstra's algorithm depends on the data structures used for implementing the priority queue. With a binary heap or Fibonacci heap, the algorithm typically runs in O((|V|+|E|) log |V|) time, where |V| represents the number of vertices and |E| represents the number of edges. However, using a binary heap implementation, the algorithm can achieve a complexity of O((|V|+|E|) log |V|) in the worst case.

Real-World Applications:
Dijkstra's algorithm has numerous applications in various domains, including:

- Routing protocols in computer networks and the internet.
- GPS navigation systems for finding the shortest path between locations.
- Network analysis and optimization for efficient data transfer.
- Recommendation systems for suggesting the most relevant items to users.
- Game development for pathfinding and AI-controlled character movement.

In conclusion, Dijkstra's algorithm is a powerful tool for finding the shortest path in a weighted graph. It efficiently utilizes priority queues and provides optimal solutions for various real-world problems. However, it is important to note that the algorithm has limitations. It assumes non-negative edge weights and may produce incorrect results when negative weights are present. Additionally, for large graphs or scenarios requiring multiple sources or destinations, alternative algorithms may be more suitable. Understanding these limitations allows for informed decision-making when applying Dijkstra's algorithm in practice.

To use graph as databases you can use PostgreSQL's extension Apache AGE: -
More about apache age here: https://age.apache.org/
Github here: https://github.com/apache/age/
To implement Dijkstra's algorithm in Apache AGE, you can use drivers given here and use AGE with programming languages such as python.: https://github.com/apache/age/tree/master/drivers