用于拓扑排序的Java程序

有向无环图 (DAG) 的拓扑排序是顶点的线性排序，因此对于每个有向边 uv，顶点 u 在排序中位于 v 之前。如果图不是 DAG，则无法对图进行拓扑排序。

例如，下图的拓扑排序是“5 4 2 3 1 0”。一个图可以有多个拓扑排序。例如，下图的另一种拓扑排序是“4 5 2 3 1 0”。拓扑排序中的第一个顶点始终是入度为 0 的顶点（没有入边的顶点）。

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 adj[]; // Adjacency List
  
    //Constructor
    Graph(int v)
    {
        V = v;
        adj = new LinkedList[v];
        for (int i=0; i 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 0Please refer complete article on Topological Sorting for more details!

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