topological sort implementation, DFS based approach

Author: NirmalSilwal

GeeksForGeeks/Graph/TopologicalSorting.java

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+package Graph;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+
+public class TopologicalSorting {
+
+	public static void main(String[] args) {
+
+		int totalVertices = 6; // test graph 1
+		// int totalVertices = 3; // test graph 2
+		ArrayList<ArrayList<Integer>> adjList = new ArrayList<ArrayList<Integer>>(totalVertices);
+		for (int i = 0; i < totalVertices; i++) {
+			adjList.add(new ArrayList<Integer>(i));
+		}
+		// test graph 1, output: 0 2 1 3 5 4
+		addEdge(adjList, 0, 1);
+		addEdge(adjList, 0, 2);
+		addEdge(adjList, 1, 3);
+		addEdge(adjList, 2, 3);
+		addEdge(adjList, 3, 4);
+		addEdge(adjList, 3, 5);
+
+		// test graph 2, output: 0 2 1
+		// addEdge(adjList, 0, 1);
+		// addEdge(adjList, 0, 2);
+
+		display(adjList);
+
+		System.out.println("\ntopological sorting...");
+		topologicalSort(adjList);
+	}
+
+	private static void addEdge(ArrayList<ArrayList<Integer>> adjList, int u, int v) {
+		adjList.get(u).add(v); // directed graph Edge
+	}
+
+	private static void display(ArrayList<ArrayList<Integer>> adjList) {
+		for (int i = 0; i < adjList.size(); i++) {
+			System.out.println(i + ": " + adjList.get(i));
+		}
+	}
+
+	// O(V+E) Time, DFS based approach
+	public static void topologicalSort(ArrayList<ArrayList<Integer>> adjList) {
+		// use addFirst() and removeFirst() method of LinkedList for O(1) Time
+		LinkedList<Integer> stack = new LinkedList<>();
+
+		boolean[] visited = new boolean[adjList.size()];
+
+		for (int vtx = 0; vtx < adjList.size(); vtx++) {
+			if (visited[vtx] == false) {
+				dfsTraverse(adjList, vtx, stack, visited);
+			}
+		}
+		
+		while (!stack.isEmpty()) {
+			System.out.print(stack.removeFirst() + " "); // pop
+		}
+	}
+
+	private static void dfsTraverse(ArrayList<ArrayList<Integer>> adjList, int vertex, LinkedList<Integer> stack,
+			boolean[] visited) {
+
+		visited[vertex] = true;
+
+		for (Integer neighbor : adjList.get(vertex)) {
+			if (visited[neighbor] == false) {
+				dfsTraverse(adjList, neighbor, stack, visited);
+			}
+		}
+		stack.addFirst(vertex); // push
+	}
+}