Java Program to Implement Tarjan Algorithm

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This is a Java Program to Implement Tarjan Algorithm. Tarjan Algorithm is used for finding all strongly connected components in a graph.

Here is the source code of the Java Program to Implement Tarjan Algorithm. The Java program is successfully compiled and run on a Windows system. The program output is also shown below.

  1. /**
  2.  *     Java Program to Implement Tarjan Algorithm
  3.  **/
  4.  
  5. import java.util.*;
  6.  
  7. /** class Tarjan **/
  8. class Tarjan
  9. {
  10.     /** number of vertices **/
  11.     private int V;    
  12.     /** preorder number counter **/
  13.     private int preCount;
  14.     /** low number of v **/
  15.     private int[] low;
  16.     /** to check if v is visited **/
  17.     private boolean[] visited;      
  18.     /** to store given graph **/
  19.     private List<Integer>[] graph;
  20.     /** to store all scc **/
  21.     private List<List<Integer>> sccComp;
  22.     private Stack<Integer> stack;
  23.  
  24.     /** function to get all strongly connected components **/
  25.     public List<List<Integer>> getSCComponents(List<Integer>[] graph) 
  26.     {
  27.         V = graph.length;
  28.         this.graph = graph;
  29.         low = new int[V];
  30.         visited = new boolean[V];
  31.         stack = new Stack<Integer>();
  32.         sccComp = new ArrayList<>();
  33.  
  34.         for (int v = 0; v < V; v++)
  35.               if (!visited[v])
  36.                 dfs(v);
  37.  
  38.         return sccComp;
  39.     }
  40.     /** function dfs **/
  41.     public void dfs(int v) 
  42.     {
  43.         low[v] = preCount++;
  44.         visited[v] = true;
  45.         stack.push(v);
  46.         int min = low[v];
  47.         for (int w : graph[v]) 
  48.         {
  49.             if (!visited[w])
  50.                 dfs(w);
  51.             if (low[w] < min) 
  52.                 min = low[w];
  53.         }
  54.         if (min < low[v]) 
  55.         { 
  56.             low[v] = min; 
  57.             return; 
  58.         }        
  59.         List<Integer> component = new ArrayList<Integer>();
  60.         int w;
  61.         do
  62.         {
  63.             w = stack.pop();
  64.             component.add(w);
  65.             low[w] = V;                
  66.         } while (w != v);
  67.         sccComp.add(component);        
  68.     }    
  69.     /** main **/
  70.     public static void main(String[] args)
  71.     {    
  72.         Scanner scan = new Scanner(System.in);
  73.         System.out.println("Tarjan algorithm Test\n");
  74.         System.out.println("Enter number of Vertices");
  75.         /** number of vertices **/
  76.         int V = scan.nextInt();
  77.  
  78.         /** make graph **/
  79.         List<Integer>[] g = new List[V];        
  80.         for (int i = 0; i < V; i++)
  81.             g[i] = new ArrayList<Integer>();        
  82.         /** accpet all edges **/
  83.         System.out.println("\nEnter number of edges");
  84.         int E = scan.nextInt();
  85.         /** all edges **/
  86.         System.out.println("Enter "+ E +" x, y coordinates");
  87.         for (int i = 0; i < E; i++)
  88.         {
  89.             int x = scan.nextInt();
  90.             int y = scan.nextInt();
  91.             g[x].add(y);
  92.         }
  93.  
  94.         Tarjan t = new Tarjan();        
  95.         System.out.println("\nSCC : ");
  96.         /** print all strongly connected components **/
  97.         List<List<Integer>> scComponents = t.getSCComponents(g);
  98.            System.out.println(scComponents);        
  99.     }    
  100. }

Tarjan algorithm Test
 
Enter number of Vertices
8
 
Enter number of edges
14
Enter 14 x, y coordinates
0 1
1 2
2 3
3 2
3 7
7 3
2 6
7 6
5 6
6 5
1 5
4 5
4 0
1 4
 
SCC :
[[5, 6], [7, 3, 2], [4, 1, 0]]

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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