## Bidirectional Search: A Graph Search Algorithm

When it comes to finding the shortest path between two points on a graph, one of the most efficient algorithms is known as Bidirectional Search. This approach has a lot of advantages over more traditional search methods, including reduced computational time and improved accuracy.

In this article, we will explore the concept of Bidirectional Search, how it works, and why it is so effective.

### What is Bidirectional Search?

Before we dive into specifics, let’s start with a broad definition of Bidirectional Search. At its core, it is a graph search algorithm that uses two search processes to find the route between two nodes on a graph.

In contrast to traditional search algorithms that begin at the starting point of a graph and systematically explore all potential paths to the destination, Bidirectional Search begins the search from both ends of the graph concurrently and only continues until they meet in the middle.

### Why is Bidirectional Search effective?

The main reason Bidirectional Search is so effective is that it cuts down on the number of nodes that need to be explored, which reduces the computational time significantly. With traditional search algorithms, the number of nodes that need to be evaluated grows exponentially as the graph’s size increases, but Bidirectional Search requires only the square root of the number of nodes, making it far more efficient.

Moreover, it is more accurate because it only considers paths that are plausible alternatives.

### How does Bidirectional Search work?

To understand how Bidirectional Search works, we need first to define the nodes and edges of a graph. The nodes represent specific points in the graph, while the edges represent the paths that lead from one node to another.

The graph can be either directed or undirected, meaning that each edge has a direction or that it goes both ways, respectively. Let us assume that we have a graph that we want to traverse from node A to node B.

Traditional searches will start exploring a path at node A, following each possible path (edge) until the path to node B is found. This approach has a significant disadvantage in that it may take a very long time to reach the destination, especially for larger graphs.

A better approach is to start searching simultaneously from both nodes A and B, so two search processes will start at the same time. Each process will explore the adjacent nodes (neighbours) of their respective node and keep track of the visited nodes.

The algorithm continues until both searches identify and evaluate the same node, which means that they have found the shortest path. The advantage of this approach is that it divides the search space in two halves, one from A to the midpoint and another from B to the midpoint, and therefore, it is more efficient since each search takes only a square root of the graph nodes.

## Examples of Bidirectional Search

Bidirectional Search can be applied in a variety of scenarios, from simple graphs to more complex ones. For example, let’s consider a simple weighted graph with five nodes labeled A, B, C, D, and E, and with the weights indicated in the figure below.

Image Source: https://en.wikipedia.org/wiki/Bidirectional_search#/media/File:Breadthfirstgraph.png

Suppose we want to find the shortest path between nodes A and D. One option is to apply Dijkstra’s algorithm, which is a traditional graph search method that uses a priority queue.

However, we can also apply Bidirectional Search to find the optimal path. We first start from A and D together and proceed to the adjacent nodes.

A explores B while D explores C since both nodes have only one neighbour. We can stop the search since these two nodes never meet (i.e., no path exists).

Then, we move the search to the next level, where A explores C and D explores B. They both meet at node C, and if we look at the figures, we can see that (A-C-D) is the optimal path.

## Final Thoughts

Bidirectional Search is an efficient approach to finding the shortest path between two points on a graph. By utilizing two search processes that start from opposite ends of the graph, this algorithm significantly reduces the computational time required for the search while also improving its accuracy.

This approach can be applied to different types of graphs, making it an incredibly versatile technique. Furthermore, since Bidirectional Search is scalable to large graphs, it is an excellent starting point in many real-world applications that require the shortest path search to be completed in a reasonable amount of time.

### 3) How does Bidirectional Search work?

To get a better understanding of how Bidirectional Search works, let’s illustrate it using an existing graph.

Suppose we have a graph with the nodes A, B, C, D, E, F, G, and H, as shown below:

Image source: https://en.wikipedia.org/wiki/Bidirectional_search#/media/File:Bidirectional_search_animation.gif

Now, let’s say we want to find the shortest path between node A and node H. With Bidirectional Search, we start by exploring nodes A and H simultaneously.

We create two sets of nodes, one that stores the explored nodes from A and the other that stores the explored nodes from H.

- Add nodes A and H to their respective explored sets.
- Expand the neighbor nodes for both A and H simultaneously.
- Since nodes B and G are adjacent to both nodes A and H, they are added to their respective explored sets.
### Check if B and G have been visited by the other search:

- If node B has been explored by H’s search, a path has been found and path reconstruction is possible from nodes A to H.
- If node G has been explored by A’s search, a path has been found and path reconstruction is possible from nodes A to H.

- If a path has not been found, we continue the search, iteratively expanding the neighbor nodes with the fewest explored paths on the current search side until a path is found.

As we continue the search, it is evident that the number of nodes to be explored is much lower than in traditional searching methods. We continue exploring the graph until we reach the same node on both paths, indicating that we have found the shortest path between A and H.

### 4) Implementing Bidirectional Search in Python

Now that we have an understanding of how Bidirectional Search works let’s explore how to implement it in Python. For this implementation, we will assume that the graph is represented as an adjacency list.

An adjacency list represents a graph as a collection of lists of the nodes’ adjacent nodes. Below is an example of a Python implementation of Bidirectional Search using adjacency lists:

```
from queue import Queue, PriorityQueue
def bidirectional_search(graph, start, end):
# Define the explored set for the start and end nodes
start_explored = {start}
end_explored = {end}
# Create queues for both search directions
start_queue = Queue()
start_queue.put(start)
end_queue = Queue()
end_queue.put(end)
# Create mappings for the start and end distance from a node
start_distance = {start: 0}
end_distance = {end: 0}
while not start_queue.empty() and not end_queue.empty():
# Start searching from the starting node
if start_queue.qsize() < end_queue.qsize():
queue = start_queue
explored = start_explored
distance = start_distance
else:
# Start searching from the ending node
queue = end_queue
explored = end_explored
distance = end_distance
node = queue.get()
# Retrieve the adjacent nodes
for neighbor in graph[node]:
if neighbor not in explored:
explored.add(neighbor)
distance[neighbor] = distance[node] + 1
queue.put(neighbor)
# Check if the adjacent node has been visited by the other end
if neighbor in start_explored:
return distance[neighbor] + start_distance[neighbor]
if neighbor in end_explored:
return distance[neighbor] + end_distance[neighbor]
return -1 # If a path is not found
```

Let’s break down the implementation.

First, we create two sets for the explored nodes for the start and end nodes. We then create queues for each search direction and map the start and end distances from each node in the graph.

We then begin the search process by entering an iterative loop. We start searching from the starting node if the starting node’s queue has fewer nodes than the ending node’s queue.

Otherwise, we start from the ending node. We then retrieve the next node in the search queue and explore all its adjacent nodes.

If we discover a new node, we add it to its corresponding explored set and update its distance map. If we encounter a node that has already been explored by the other search, we return the sum of the distances from both ends to that node.

If a path is not found, the search loop continues.

Finally, if a path is not found, the function returns -1.

## Conclusion

Bidirectional Search has proven to be an excellent algorithm for finding the shortest path between two nodes on a graph. It significantly reduces the computational time required for the search, making it an excellent choice for large graphs.

Furthermore, it is considerably more efficient in terms of exploring nodes in potential paths than traditional search algorithms, reducing the number of nodes to be searched and saving time. With the Python implementation provided, this algorithm can be implemented in many applications that require the shortest path search to be completed in a short amount of time.

### 5) Complexity of Bidirectional Search

The time complexity of Bidirectional Search is significantly less than that of traditional search algorithms. Consider a graph with N nodes.

With traditional search, we would need to explore up to 2^N paths between the source and destination nodes. On the other hand, the Bidirectional Search algorithm’s time complexity is the square root of 2^N.

Thus, the time complexity of Bidirectional Search is O(sqrt(N)), which is significantly less than the O(N) time complexity of traditional search algorithms. Therefore, Bidirectional Search is a more efficient algorithm for finding the shortest path between two nodes on large graphs.

### 6) Advantages

#### Bidirectional Search has several advantages over traditional search algorithms:

**Speed:**Bidirectional Search typically has a more rapid search process since it searches from both the source and destination simultaneously, reducing the number of paths needed to be traversed.Moreover, since it deals with the square root of the graph nodes, it is much faster than traditional search algorithms.

**Resource Conservation:**As the number of nodes to be traversed in Bidirectional Search is much less than in traditional search algorithms, the memory and computational resources used during the search process are minimal, leading to improved performance.**Optimal Path Finding:**Bidirectional Search can find the optimal path between two nodes in the graph because search paths converge in the middle of the graph. Therefore, it can always obtain the shortest path between the source and the destination node.**Scalability:**Bidirectional Search is scalable and can handle graphs of any size without sacrificing efficiency.Moreover, since it involves searching from both ends, it is the best option in cases that require finding paths in large graphs.

**Reduced Search Space:**Bidirectional Search reduces the search space by searching from both ends, halving the number of states to explore. In some cases, this reduction in search space can result in an exponential improvement in search speed.

## Conclusion

Bidirectional Search is an efficient algorithm for finding the shortest path between two nodes on a graph. It has a much faster search process than traditional search algorithms and uses minimal memory and computational resources.

Moreover, it can find the optimal path, is scalable, and reduces the search space, making it an excellent choice for large graphs. The time complexity of Bidirectional Search is the square root of 2^N, making it much faster than traditional search algorithms.

In conclusion, Bidirectional Search is a reliable optimization that can be utilized in many real-world applications of graph search, leading to improved speed, reduced resource consumption, and optimal path generation.

### 7) Disadvantages

Although Bidirectional Search has several advantages over traditional search algorithms, it is not immune to some drawbacks and challenges. Below are some of the common disadvantages of Bidirectional Search:

**Infinite Looping:**One of the significant disadvantages of Bidirectional Search is the possibility of entering an infinite loop, which can happen when it is repeatedly explored without success. This can result in the algorithm running indefinitely, consuming resources without generating a solution.**Complexity:**Although Bidirectional Search has a square root reduced search space when compared to traditional search algorithms, it is still a complex algorithm that can be challenging to implement and optimize.**Memory:**Bidirectional Search requires the storage of multiple data structures simultaneously for the two searches to work efficiently, which can be a memory-intensive process when dealing with large graphs.**Unidirectionality:**Bidirectional Search doesn’t handle a situation where one of the nodes is identified as the obstacle, meaning it can not go further.A workaround to this would be to adopt algorithms that deal with this limitation where such issues often arise.

**Initial configurations:**Another drawback of Bidirectional Search is that it requires a defined initial configuration – a source and a destination node – making it less ideal than traditional algorithms when the start and end nodes are not known in advance.

### 8) Summary

Bidirectional Search is an efficient search algorithm that can find the shortest path between two nodes on a graph. Unlike traditional search algorithms, which traverse all possible paths from one end to the other, Bidirectional Search reduces the search space by conducting searches from both ends.

This approach minimizes the time and memory required to find the optimal path between two nodes. Bidirectional Search is scalable, more accurate, and faster than traditional search algorithms.

It is ideal for large graphs and time-critical applications. With Python implementations and efficient data structures, the algorithm can be further optimized to reduce memory usage and enhance its performance.

However, Bidirectional Search is not without drawbacks. There are possibilities of infinite loops, initial configurations requirements, and unidirectional limitations that could limit its application.

In conclusion, Bidirectional Search is a valuable optimization for problems that require finding paths in a large graph and reducing computation time accurately. When used correctly and optimized to address possible challenges, Bidirectional Search can become a powerful tool for graph search operations.

Bidirectional Search is an algorithm that significantly reduces the time required to find the shortest path between two nodes on a graph. Traditional search algorithms can have a time complexity of O(N), while Bidirectional Search has a complexity of O(sqrt(N)).

The algorithm is scalable, more accurate, and takes less computational time. Bidirectional Search has its limitations such as data storage requirements, initial configuration demands, and infinite loop possibilities but it is still a worthwhile optimization for graph searches.

With efficient implementations, it can be an excellent tool for solving problems in time-critical applications that require the shortest path to be found.