Breadth First Traversal (or Search) for a graph is similar to Breadth First Traversal of a tree (See method 2 of this post). The only catch here is, unlike trees, graphs may contain cycles, so we may come to the same node again. To avoid processing a node more than once, we use a boolean visited array. For simplicity, it is assumed that all vertices are reachable from the starting vertex.
For example, in the following graph, we start traversal from vertex 2. When we come to vertex 0, we look for all adjacent vertices of it. 2 is also an adjacent vertex of 0. If we don’t mark visited vertices, then 2 will be processed again and it will become a non-terminating process. A Breadth First Traversal of the following graph is 2, 0, 3, 1.
Following are the implementations of simple Breadth First Traversal from a given source.
// Mark the current node as visited and enqueue it
while(queue.size() != 0)
// Dequeue a vertex from queue and print it
s = queue.poll();
// Get all adjacent vertices of the dequeued vertex s
// If a adjacent has not been visited, then mark it
// visited and enqueue it
Iterator<Integer> i = adj[s].listIterator();
intn = i.next();
visited[n] = true;
// Driver method to
Graph g = newGraph(4);
System.out.println("Following is Breadth First Traversal "+
"(starting from vertex 2)");
// This code is contributed by Aakash Hasija
# Python3 Program to print BFS traversal
# from a given source vertex. BFS(int s)
# traverses vertices reachable from s.
# This class represents a directed graph
# using adjacency list representation
# default dictionary to store graph
# function to add an edge to graph
# Function to print a BFS of graph
# Mark all the vertices as not visited
visited =[False] *(len(self.graph))
# Create a queue for BFS
# Mark the source node as
# visited and enqueue it
# Dequeue a vertex from
# queue and print it
print(s, end =" ")
# Get all adjacent vertices of the
# dequeued vertex s. If a adjacent
# has not been visited, then mark it
# visited and enqueue it
# Driver code
# Create a graph given in
# the above diagram
print("Following is Breadth First Traversal"
" (starting from vertex 2)")
# This code is contributed by Neelam Yadav
Following is Breadth First Traversal (starting from vertex 2)
2 0 3 1
Note that the above code traverses only the vertices reachable from a given source vertex. All the vertices may not be reachable from a given vertex (example Disconnected graph). To print all the vertices, we can modify the BFS function to do traversal starting from all nodes one by one (Like the DFS modified version) .
Time Complexity: O(V+E) where V is number of vertices in the graph and E is number of edges in the graph.