This is a java program to check whether graph is DAG. In mathematics and computer science, a directed acyclic graph (DAG Listeni/’dæg/), is a directed graph with no directed cycles. That is, it is formed by a collection of vertices and directed edges, each edge connecting one vertex to another, such that there is no way to start at some vertex v and follow a sequence of edges that eventually loops back to v again.

Here is the source code of the Java Program to Check Whether Graph is DAG. The Java program is successfully compiled and run on a Windows system. The program output is also shown below.

package com.sanfoundry.hardgraph;

import java.util.HashMap;

import java.util.Iterator;

import java.util.LinkedList;

import java.util.List;

import java.util.Map;

import java.util.Scanner;

`class GraphLinkedList`

`{`

private Map<Integer, List<Integer>> adjacencyList;

public GraphLinkedList(int v)

`{`

adjacencyList = new HashMap<Integer, List<Integer>>();

for (int i = 1; i <= v; i++)

adjacencyList.put(i, new LinkedList<Integer>());

`}`

public void setEdge(int from, int to)

`{`

if (to > adjacencyList.size() || from > adjacencyList.size())

System.out.println("The vertices does not exists");

`/*`

`* List<Integer> sls = adjacencyList.get(to);`

`* sls.add(from);`

`*/`

List<Integer> dls = adjacencyList.get(from);

dls.add(to);

`}`

public List<Integer> getEdge(int to)

`{`

if (to > adjacencyList.size())

`{`

System.out.println("The vertices does not exists");

return null;

`}`

return adjacencyList.get(to);

`}`

public boolean checkDAG()

`{`

Integer count = 0;

Iterator<Integer> iteratorI = this.adjacencyList.keySet().iterator();

Integer size = this.adjacencyList.size() - 1;

while (iteratorI.hasNext())

`{`

Integer i = iteratorI.next();

List<Integer> adjList = this.adjacencyList.get(i);

if (count == size)

`{`

return true;

`}`

if (adjList.size() == 0)

`{`

`count++;`

System.out.println("Target Node - " + i);

Iterator<Integer> iteratorJ = this.adjacencyList.keySet()

.iterator();

while (iteratorJ.hasNext())

`{`

Integer j = iteratorJ.next();

List<Integer> li = this.adjacencyList.get(j);

if (li.contains(i))

`{`

li.remove(i);

System.out.println("Deleting edge between target node "

+ i + " - " + j + " ");

`}`

`}`

this.adjacencyList.remove(i);

iteratorI = this.adjacencyList.keySet().iterator();

`}`

`}`

return false;

`}`

public void printGraph()

`{`

System.out.println("The Graph is: ");

for (int i = 1; i <= this.adjacencyList.size(); i++)

`{`

List<Integer> edgeList = this.getEdge(i);

if (edgeList.size() != 0)

`{`

System.out.print(i);

for (int j = 0; j < edgeList.size(); j++)

`{`

System.out.print(" -> " + edgeList.get(j));

`}`

System.out.println();

`}`

`}`

`}`

`}`

public class CheckDAG

`{`

public static void main(String args[])

`{`

int v, e, count = 1, to, from;

Scanner sc = new Scanner(System.in);

`GraphLinkedList glist;`

`try`

`{`

System.out.println("Enter the number of vertices: ");

v = sc.nextInt();

System.out.println("Enter the number of edges: ");

e = sc.nextInt();

glist = new GraphLinkedList(v);

System.out.println("Enter the edges in the graph : <from> <to>");

while (count <= e)

`{`

to = sc.nextInt();

from = sc.nextInt();

glist.setEdge(to, from);

`count++;`

`}`

glist.printGraph();

System.out

.println("--Processing graph to check whether it is DAG--");

if (glist.checkDAG())

`{`

System.out

.println("Result: \nGiven graph is DAG (Directed Acyclic Graph).");

`}`

`else`

`{`

System.out

.println("Result: \nGiven graph is not DAG (Directed Acyclic Graph).");

`}`

`}`

catch (Exception E)

`{`

System.out

.println("You are trying to access empty adjacency list of a node.");

`}`

sc.close();

`}`

`}`

Output:

$ javac CheckDAG.java $ java CheckDAG Enter the number of vertices: 6 Enter the number of edges: 7 Enter the edges in the graph : <from> <to> 1 2 2 3 2 4 4 5 4 6 5 6 6 3 The Graph is: 1 -> 2 2 -> 3 -> 4 4 -> 5 -> 6 5 -> 6 6 -> 3 --Processing graph to check whether it is DAG-- Target Node - 3 Deleting edge between target node 3 - 2 Deleting edge between target node 3 - 6 Target Node - 6 Deleting edge between target node 6 - 4 Deleting edge between target node 6 - 5 Target Node - 5 Deleting edge between target node 5 - 4 Target Node - 4 Deleting edge between target node 4 - 2 Target Node - 2 Deleting edge between target node 2 - 1 Result: Given graph is DAG (Directed Acyclic Graph). Enter the number of vertices: 6 Enter the number of edges: 7 Enter the edges in the graph : <from> <to> 1 2 2 3 2 4 4 5 5 6 6 4 6 3 The Graph is: 1 -> 2 2 -> 3 -> 4 4 -> 5 5 -> 6 6 -> 4 -> 3 --Processing graph to check whether it is DAG-- Target Node - 3 Deleting edge between target node 3 - 2 Deleting edge between target node 3 - 6 Result: Given graph is not DAG (Directed Acyclic Graph).

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