Graph – Find Number of non reachable vertices from a given vertex

Objective: Given undirected graph write an algorithm to count the number of non reachable vertices from a given vertex

Example:

Approach:

Use DFS

Do the DFS (Depth-First Search) from the given vertex A, Recursively do the DFS from all the adjacent vertices of given vertex A.
Use the visited array to keep track of visited vertices.
Once the DFS is completed, count the number of false in visited array. These are the vertices which did not get visit.

Code:

	import java.util.LinkedList;

	public class NonReachableVertices {
	static class Graph{
	int vertices;
	LinkedList<Integer>[] adjList;

	Graph(int vertices){
	this.vertices = vertices;
	adjList = new LinkedList[vertices];
	for (int i = 0; i <vertices ; i++) {
	adjList[i] = new LinkedList<>();
	}
	}
	public void addEgde(int source, int destination){
	adjList[source].addFirst(destination);

	adjList[destination].addFirst(source);
	}

	public int calculateVertices(int vertex){
	boolean [] visited = new boolean[vertices];
	//Do the DFS from the given vertex
	dfs(vertex, visited);

	//count the number of non reached vertices
	int count = 0;
	for (int i = 0; i <visited.length ; i++) {
	if(visited[i]==false)
	count++;
	}
	return count;
	}

	public void dfs(int start, boolean [] visited){
	visited[start] = true;
	for (int i = 0; i <adjList[start].size() ; i++) {
	int vertex = adjList[start].get(i);
	if(!visited[vertex])
	dfs(vertex,visited);
	}
	}
	}

	public static void main(String[] args) {
	int vertices = 8;
	Graph graph = new Graph(vertices);
	graph.addEgde(0, 1);
	graph.addEgde(1, 2);
	graph.addEgde(2, 3);
	graph.addEgde(3, 1);
	graph.addEgde(4, 5);
	graph.addEgde(5, 6);
	int nonReachableVertices = graph.calculateVertices(0);
	System.out.println("Number of non reachable vertices from the vertex 0 are: " + nonReachableVertices);
	nonReachableVertices = graph.calculateVertices(5);
	System.out.println("Number of non reachable vertices from the vertex 6 are: " + nonReachableVertices);
	}
	}

Output:

Number of non reachable vertices from the vertex 0 are: 4
Number of non reachable vertices from the vertex 6 are: 5