If you want to practice data structure and algorithm programs, you can go through data structure and algorithm interview questions.

We have already seen about breadth first search in level order traversal of binary tree.

### Graph traversal Algorithms:

Breadth first search is graph traversal algorithm. In this algorithm, lets say we start with node i, then we will visit neighbours of i, then neighbours of neighbours of i and so on.
It is very much similar to which is used in binary tree. We use queue to traverse graph. We put first node in queue . It repeatedly extracts node and put its neighbours in the queue. Only difference with respect to binary tree is that we need to track if node have been visited before or not. It can be easily done with help of boolean variable visited in the node. If node have been already visited then we won't visit it again.

#### Algorithm:

- Create empty queue and push root node to it.
- Do the following when queue is not empty
- Pop a node from queue and print it.
- Find neighbours of node with the help of adjacency matrix and check if node is already visited or not.
- Push neighbours of node into queue if not null

Lets say graph is:

import java.util.ArrayList; import java.util.LinkedList; import java.util.Queue; public class BreadthFirstSearchExample { private Queue<Node> queue; static ArrayList<Node> nodes=new ArrayList<Node>(); static class Node { int data; boolean visited; Node(int data) { this.data=data; } } public BreadthFirstSearchExample() { queue = new LinkedList<Node>(); } // find neighbors of node using adjacency matrix // if adjacency_matrix[i][j]==1, then nodes at index i and index j are connected public ArrayList<Node> findNeighbours(int adjacency_matrix[][],Node x) { int nodeIndex=-1; ArrayList<Node> neighbours=new ArrayList<Node>(); for (int i = 0; i < nodes.size(); i++) { if(nodes.get(i).equals(x)) { nodeIndex=i; break; } } if(nodeIndex!=-1) { for (int j = 0; j < adjacency_matrix[nodeIndex].length; j++) { if(adjacency_matrix[nodeIndex][j]==1) { neighbours.add(nodes.get(j)); } } } return neighbours; } public void bfs(int adjacency_matrix[][], Node node) { queue.add(node); node.visited=true; while (!queue.isEmpty()) { Node element=queue.remove(); System.out.print(element.data + "\t"); ArrayList<Node> neighbours=findNeighbours(adjacency_matrix,element); for (int i = 0; i < neighbours.size(); i++) { Node n=neighbours.get(i); if(n!=null && !n.visited) { queue.add(n); n.visited=true; } } } } public static void main(String arg[]) { Node node40 =new Node(40); Node node10 =new Node(10); Node node20 =new Node(20); Node node30 =new Node(30); Node node60 =new Node(60); Node node50 =new Node(50); Node node70 =new Node(70); nodes.add(node40); nodes.add(node10); nodes.add(node20); nodes.add(node30); nodes.add(node60); nodes.add(node50); nodes.add(node70); int adjacency_matrix[][] = { {0,1,1,0,0,0,0}, // Node 1: 40 {0,0,0,1,0,0,0}, // Node 2 :10 {0,1,0,1,1,1,0}, // Node 3: 20 {0,0,0,0,1,0,0}, // Node 4: 30 {0,0,0,0,0,0,1}, // Node 5: 60 {0,0,0,0,0,0,1}, // Node 6: 50 {0,0,0,0,0,0,0}, // Node 7: 70 }; System.out.println("The BFS traversal of the graph is "); BreadthFirstSearchExample bfsExample = new BreadthFirstSearchExample(); bfsExample.bfs(adjacency_matrix, node40); } }When you run above program, you will get below output:

The BFS traversal of the graph is 40 10 20 30 60 50 70Please go through Algorithm Interview Programs in java for more such programs.