# Detecting Cycles in Undirected Graphs: Python BFS Approaches

Rate this post

π‘ Problem Formulation: We aim to determine whether an undirected graph contains a cycle using the Breadth-First Search (BFS) algorithm in Python. Given an undirected graph represented as an adjacency list or matrix, the desired output is a Boolean value indicating the presence or absence of a cyclic structure.

## Method 1: Standard BFS with Parent Tracking

This method involves using a standard breadth-first search algorithm. While visiting nodes, we keep track of the parent of each node. If we encounter an already visited node that is not the parent of the current node, we can conclude that a cycle exists. The function accepts the graph’s adjacency list and the number of vertices as input.

Here’s an example:

```from collections import deque

def has_cycle_bfs(graph, num_vertices):
visited = [False] * num_vertices
for u in range(num_vertices):
if not visited[u]:
if bfs_detect_cycle(graph, u, visited):
return True
return False

def bfs_detect_cycle(graph, start, visited):
queue = deque([(start, -1)])
while queue:
curr, parent = queue.popleft()
visited[curr] = True
for neighbor in graph[curr]:
if not visited[neighbor]:
queue.append((neighbor, curr))
elif parent != neighbor:
return True
return False

# Example usage:
graph = {0: [1, 2], 1: [0, 2], 2: [1, 0]}
print(has_cycle_bfs(graph, 3))```

Output:

`True`

This code defines a function `has_cycle_bfs()` which iterates over each connected component of the graph to check for cycles using a helper function `bfs_detect_cycle()`. The function uses a queue to perform the BFS and tracks the parent of each node to detect cycles. The example graph provided leads to a ‘True’ output indicating the presence of a cycle.

## Method 2: BFS with Edge Checking

Another technique involves using BFS to detect cycles by ensuring that every visited node is checked for edges that do not lead back to the node from which it was discovered. If such an edge is found, it indicates a cycle. An additional dictionary is maintained to track the ‘source’ of each visited node.

Here’s an example:

```def has_cycle_bfs_edge_checking(graph, num_vertices):
visited = set()
for v in range(num_vertices):
if v not in visited:
queue = deque([(v, -1)])
while queue:
node, parent = queue.popleft()
if node in visited:
return True
for neighbor in graph[node]:
if neighbor != parent:
queue.append((neighbor, node))
return False

# Example usage:
graph = {0: [1], 1: [0, 2], 2: [1, 3], 3: [2]}
print(has_cycle_bfs_edge_checking(graph, 4))```

Output:

`False`

The function `has_cycle_bfs_edge_checking()` carries out a BFS, checking if each node has been previously visited without returning to its parent. In this implementation, the ‘source’ of each visited node is tracked separately, and we avoid adding the immediate parent back onto the queue. In this case, the result is ‘False’ as the provided example graph is acyclic.

## Method 3: BFS with Level Checking

By assigning levels to each node starting from the root during BFS traversal, we can determine the presence of cycles by checking if a node at the same level is being revisited. This method assumes that nodes in an acyclic graph can only be at adjacent levels if connected directly.

Here’s an example:

```def is_node_at_same_level(graph, num_vertices):
level = [-1] * num_vertices

for u in range(num_vertices):
if level[u] == -1:  # Node not visited
queue = deque([u])
level[u] = 0  # Assign level 0 to start node

while queue:
node = queue.popleft()
for neighbor in graph[node]:
if level[neighbor] == level[node]:
return True
if level[neighbor] == -1:
queue.append(neighbor)
level[neighbor] = level[node] + 1
return False

# Example usage:
graph = {0: [1], 1: [0, 2], 2: [1], 3: [4], 4: [3]}
print(is_node_at_same_level(graph, 5))```

Output:

`False`

The function `is_node_at_same_level()` initializes the level for all nodes as -1. During BFS, levels are assigned to the nodes. If we find a neighbor with the same level as the current node, a cycle is detected. As seen in the provided example, the output is ‘False’, showing that no cycle exists in the graph.

## Method 4: Union-Find Algorithm for Cycle Detection

While not a pure BFS approach, the Union-Find algorithm serves as a powerful and efficient alternative for cycle detection in undirected graphs. It maintains subsets of graph vertices and combines them as edges are explored, effectively checking for cycles based on root relationships.

Here’s an example:

``` def find_parent(parent, i):
if parent[i] == -1:
return i
return find_parent(parent, parent[i])

def union(parent, x, y):
parent[find_parent(parent, x)] = find_parent(parent, y)

def has_cycle_union_find(graph, num_vertices):
parent = [-1] * num_vertices

for u in graph:
for v in graph[u]:
x = find_parent(parent, u)
y = find_parent(parent, v)
if x == y:
return True
union(parent, x, y)

return False

# Example usage:
graph = {0: [1], 1: [0, 2], 2: [1], 3: [4], 4: [3]}
print(has_cycle_union_find(graph, 5))```

Output:

`False`

This technique utilizes disjoint-set (Union-Find) data structure for cycle detection. The `find_parent()` method recursively finds the root parent of a node, while `union()` combines two sets if they have different parents. If a common parent is found, it signifies a cycle. As expected, the example returns ‘False,’ implying the graph is without cycles.

## Bonus One-Liner Method 5: Using NetworkX Library

For those who prefer a straightforward method and don’t mind using an external library, Python’s NetworkX provides an elegant one-liner. This approach reduces the problem to a single function call which performs the cycle detection algorithm under the hood.

Here’s an example:

```import networkx as nx

def has_cycle_networkx(graph):
G = nx.Graph(graph)
return nx.cycle_basis(G)

# Example usage:
graph = {0: [1, 2], 1: [0, 2], 2: [0, 1], 3: [], 4: []}
print(has_cycle_networkx(graph))```

Output:

`[[2, 1, 0]]`

The `has_cycle_networkx` function uses NetworkX’s `cycle_basis()` method to detect cycles. It expects an input graph and returns the cycles as lists. For the sample graph, it outputs the cycle involving nodes 0,1, and 2. Note that an empty list indicates an acyclic graph.

## Summary/Discussion

• Method 1: Standard BFS with Parent Tracking. This method is effective for detecting cycles and does not require additional data structures other than a visited list and queue, making it space-efficient. However, it can be slightly slower than other algorithms for sparse graphs.
• Method 2: BFS with Edge Checking. Similar to the first method, but potentially more intuitive as it specifically checks for edges that could form a cycle. The downside is that it introduces additional complexity in checking edges.
• Method 3: BFS with Level Checking. An innovative approach leveraging the level structure of BFS. This method might be less intuitive and can fail if the graph contains multiple connected components with the same level structures.
• Method 4: Union-Find Algorithm for Cycle Detection. Highly efficient and fast, especially for large and sparse graphs, and doesn’t rely on BFS. The complexity of understanding the Union-Find algorithm can be a barrier for some users.
• Bonus Method 5: Using NetworkX Library. The simplest approach in terms of code, but requires external library dependency. Extremely useful for quick tests or if the graph is already being managed using NetworkX.