# What is graph traversal?

by Stephen M. Walker II, Co-Founder / CEO

## What is graph traversal?

Graph traversal, also known as graph search, is a process in computer science that involves visiting each vertex in a graph. This process is categorized based on the order in which the vertices are visited.

There are two primary methods for graph traversal: Depth-First Search (DFS) and Breadth-First Search (BFS).

DFS starts at a root node and explores as far as possible along each branch before backtracking. It uses a stack data structure to remember to get the next vertex to start a search when a dead end occurs in any iteration.

BFS, on the other hand, starts at the root node and visits all the sibling vertices before visiting the child vertices. It uses a queue data structure.

Graph traversal is a fundamental concept in graph theory and has numerous applications. It's used in machine learning to represent relationships between data points and make predictions. It's also used in pathfinding, which involves finding a path from one point to another, and in search algorithms, which may use graph traversal to find the best path from a starting point to a goal. Other applications include network analysis, route finding, game theory, and more.

One challenge with graph traversal is that the graph may be too large to fit into memory, making traversal difficult or impossible. Additionally, the graph may be too complex, making it hard to find an efficient path. Lastly, the graph may be dynamic, meaning that nodes and edges can be added or removed, which can make traversal more difficult.

Despite these challenges, graph traversal remains a powerful tool in computer science and artificial intelligence, providing a foundation for many complex algorithms and applications.

## What are some common graph traversal algorithms?

Graph traversal algorithms are techniques used to visit or check each vertex in a graph. The most common types of graph traversal algorithms are:

1. Depth-First Search (DFS) — DFS starts at a root node and explores as far as possible along each branch before backtracking. It uses a stack data structure to remember to get the next vertex to start a search when a dead end occurs in any iteration.

2. Breadth-First Search (BFS) — BFS starts at the root node and visits all the sibling vertices before visiting the child vertices. It uses a queue data structure.

In addition to DFS and BFS, there are other graph traversal algorithms, including:

1. Dijkstra's Algorithm — This is a single-source shortest path algorithm. It finds the shortest path from a given source vertex to all other vertices in the graph.

2. Kruskal's Algorithm — This algorithm finds a minimum spanning tree for a connected weighted graph. It adds increasing cost arcs at each step, skipping those whose addition would form a cycle, until a spanning tree is formed.

3. Bellman-Ford Algorithm — This algorithm computes shortest paths from a single source vertex to all of the other vertices in a weighted digraph. It is slower than Dijkstra's algorithm for the same problem, but more versatile, as it is capable of handling graphs in which some of the edge weights are negative.

4. Floyd Warshall Algorithm — This algorithm is used to find shortest paths in a weighted graph with positive or negative edge weights (but no negative cycles).

5. Johnson's Algorithm — This is a way to find the shortest paths between all pairs of vertices in an edge-weighted, directed graph. It allows some of the edge weights to be negative numbers, but no negative-weight cycles may exist.

Each of these algorithms has its own use cases and is chosen based on the specific requirements of the problem at hand.

## What is the best way to traverse a graph?

The best way to traverse a graph depends on the specific requirements of the problem you're trying to solve. Two of the most common graph traversal algorithms are Depth-First Search (DFS) and Breadth-First Search (BFS).

DFS explores as far as possible along each branch before backtracking. It uses a stack data structure, following a Last-In-First-Out (LIFO) principle. DFS is particularly useful when you need to traverse through a deep graph or tree, or when you need to find a path to a far-off node. It's also faster than BFS and requires less memory space. However, DFS can get trapped in infinite loops and its traversal order can vary depending on the order of the edges.

BFS, on the other hand, explores all the neighbors at the present depth before moving on to nodes at the next depth level. It uses a queue data structure, following a First-In-First-Out (FIFO) principle. BFS is optimal for finding the shortest path in an unweighted graph or when the target is closer to the source. However, BFS requires more memory space and is slower than DFS.

Both DFS and BFS have a time complexity of O(V + E) when an adjacency list is used, and O(V^2) when an adjacency matrix is used, where V stands for vertices and E stands for edges.

In Python, you can implement these algorithms using dictionaries to represent the graph and sets to keep track of visited nodes. Here's a simplified pseudocode for both algorithms:

DFS:

``````def dfs(graph, node, visited=None):
if visited is None:
visited = set()
if node not in visited:
for neighbour in graph[node]:
dfs(graph, neighbour, visited)``````

BFS:

``````from collections import deque

def bfs(graph, root):
visited = set()
queue = deque([root])
while queue:
vertex = queue.popleft()
if vertex not in visited:
queue.extend(graph[vertex] - visited)
return visited``````

Remember to choose the appropriate traversal algorithm based on your specific needs, such as the structure of your graph, the nature of the problem you're solving, and the resources available to you.

## Benefits, Challenges, and Applications of Graph Traversal in AI

Graph traversal is a key technique in AI with several advantages. It enables the discovery of the shortest or least resource-intensive paths between nodes, which is crucial in optimizing routes or operations. However, graph traversal also presents challenges, such as handling large graphs that exceed memory capacity, navigating complex graphs to find efficient paths, and adapting to dynamic graphs where nodes and edges frequently change.

In terms of applications, graph traversal is fundamental in pathfinding, which involves calculating the most efficient route between two points, often used in mapping and navigation systems. It's also integral to search algorithms that identify optimal paths for planning and problem-solving in various AI applications. Furthermore, in machine learning, graphs represent relationships between data points, and traversing these graphs allows algorithms to learn and make predictions. Thus, graph traversal is a versatile tool with a wide range of uses in artificial intelligence.

## More terms

### What is temporal difference learning?

Temporal Difference (TD) learning is a class of model-free reinforcement learning methods. These methods sample from the environment, similar to Monte Carlo methods, and perform updates based on current estimates, akin to dynamic programming methods. Unlike Monte Carlo methods, which adjust their estimates only once the final outcome is known, TD methods adjust predictions to match later, more accurate predictions.

### What is an evolutionary algorithm?

An evolutionary algorithm (EA) is a type of artificial intelligence-based computational method that solves problems by mimicking biological evolution processes such as reproduction, mutation, recombination, and selection. EAs are a subset of evolutionary computation and are considered a generic population-based metaheuristic optimization algorithm.