Java Program for Topological Sorting
Last Updated :
17 Jan, 2018
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Topological sorting for Directed Acyclic Graph (DAG) is a linear ordering of vertices such that for every directed edge uv, vertex u comes before v in the ordering. Topological Sorting for a graph is not possible if the graph is not a DAG.
For example, a topological sorting of the following graph is “5 4 2 3 1 0”. There can be more than one topological sorting for a graph. For example, another topological sorting of the following graph is “4 5 2 3 1 0”. The first vertex in topological sorting is always a vertex with in-degree as 0 (a vertex with no in-coming edges).
Java
// A Java program to print topological sorting of a DAG import java.io.*; import java.util.*; // This class represents a directed graph using adjacency // list representation class Graph { private int V; // No. of vertices private LinkedList<Integer> adj[]; // Adjacency List //Constructor Graph(int v) { V = v; adj = new LinkedList[v]; for (int i=0; i<v; ++i) adj[i] = new LinkedList(); } // Function to add an edge into the graph void addEdge(int v,int w) { adj[v].add(w); } // A recursive function used by topologicalSort void topologicalSortUtil(int v, boolean visited[], Stack stack) { // Mark the current node as visited. visited[v] = true; Integer i; // Recur for all the vertices adjacent to this // vertex Iterator<Integer> it = adj[v].iterator(); while (it.hasNext()) { i = it.next(); if (!visited[i]) topologicalSortUtil(i, visited, stack); } // Push current vertex to stack which stores result stack.push(new Integer(v)); } // The function to do Topological Sort. It uses // recursive topologicalSortUtil() void topologicalSort() { Stack stack = new Stack(); // Mark all the vertices as not visited boolean visited[] = new boolean[V]; for (int i = 0; i < V; i++) visited[i] = false; // Call the recursive helper function to store // Topological Sort starting from all vertices // one by one for (int i = 0; i < V; i++) if (visited[i] == false) topologicalSortUtil(i, visited, stack); // Print contents of stack while (stack.empty()==false) System.out.print(stack.pop() + " "); } // Driver method public static void main(String args[]) { // Create a graph given in the above diagram Graph g = new Graph(6); g.addEdge(5, 2); g.addEdge(5, 0); g.addEdge(4, 0); g.addEdge(4, 1); g.addEdge(2, 3); g.addEdge(3, 1); System.out.println("Following is a Topological " + "sort of the given graph"); g.topologicalSort(); } } // This code is contributed by Aakash Hasija |
Output:
Following is a Topological Sort of the given graph 5 4 2 3 1 0
Please refer complete article on Topological Sorting for more details!
