Breadth-first search (iterative)

from collections import deque

def iterative_bfs(graph, start):
    visited = set() # Set to keep track of visited nodes.
    queue = deque([start]) # Initialize a queue with the starting vertex

    while queue: # While the queue is not empty
        vertex = queue.popleft() # Remove the vertex from the queue
        if vertex not in visited:
            visited.add(vertex) # Mark the vertex as visited
            print(vertex) # Process the vertex (for example, print it)

            # Add all unvisited neighbors to the queue
            for neighbor in graph[vertex]:
                if neighbor not in visited:
                    queue.append(neighbor)

# Example usage:
# Define a graph as an adjacency list
graph = {
    1 : [2,3],
    2 : [4, 5],
    3 : [5],
    4 : [],
    5 : [1],
}

iterative_bfs(graph, 1) # Start BFS from vertex 1