# Transitive closure of a graph

• Difficulty Level : Medium
• Last Updated : 20 Jul, 2021

Given a directed graph, find out if a vertex j is reachable from another vertex i for all vertex pairs (i, j) in the given graph. Here reachable mean that there is a path from vertex i to j. The reach-ability matrix is called the transitive closure of a graph.

`For example, consider below graph` Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.

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```Transitive closure of above graphs is
1 1 1 1
1 1 1 1
1 1 1 1
0 0 0 1```

The graph is given in the form of adjacency matrix say ‘graph[V][V]’ where graph[i][j] is 1 if there is an edge from vertex i to vertex j or i is equal to j, otherwise graph[i][j] is 0.
Floyd Warshall Algorithm can be used, we can calculate the distance matrix dist[V][V] using Floyd Warshall, if dist[i][j] is infinite, then j is not reachable from I. Otherwise, j is reachable and the value of dist[i][j] will be less than V.
Instead of directly using Floyd Warshall, we can optimize it in terms of space and time, for this particular problem. Following are the optimizations:

1. Instead of an integer resultant matrix (dist[V][V] in floyd warshall), we can create a boolean reach-ability matrix reach[V][V] (we save space). The value reach[i][j] will be 1 if j is reachable from i, otherwise 0.
2. Instead of using arithmetic operations, we can use logical operations. For arithmetic operation ‘+’, logical and ‘&&’ is used, and for a min, logical or ‘||’ is used. (We save time by a constant factor. Time complexity is the same though)

Below is the implementation of the above approach:

## C++

 `// Program for transitive closure``// using Floyd Warshall Algorithm``#include` `// Number of vertices in the graph``#define V 4` `// A function to print the solution matrix``void` `printSolution(``int` `reach[][V]);` `// Prints transitive closure of graph[][]``// using Floyd Warshall algorithm``void` `transitiveClosure(``int` `graph[][V])``{``    ``/* reach[][] will be the output matrix``    ``// that will finally have the``       ``shortest distances between``       ``every pair of vertices */``    ``int` `reach[V][V], i, j, k;` `    ``/* Initialize the solution matrix same``    ``as input graph matrix. Or``       ``we can say the initial values of``       ``shortest distances are based``       ``on shortest paths considering``       ``no intermediate vertex. */``    ``for` `(i = 0; i < V; i++)``        ``for` `(j = 0; j < V; j++)``            ``reach[i][j] = graph[i][j];` `    ``/* Add all vertices one by one to the``    ``set of intermediate vertices.``      ``---> Before start of a iteration,``           ``we have reachability values for``           ``all pairs of vertices such that``           ``the reachability values``           ``consider only the vertices in``           ``set {0, 1, 2, .. k-1} as``           ``intermediate vertices.``     ``----> After the end of a iteration,``           ``vertex no. k is added to the``            ``set of intermediate vertices``            ``and the set becomes {0, 1, .. k} */``    ``for` `(k = 0; k < V; k++)``    ``{``        ``// Pick all vertices as``        ``// source one by one``        ``for` `(i = 0; i < V; i++)``        ``{``            ``// Pick all vertices as``            ``// destination for the``            ``// above picked source``            ``for` `(j = 0; j < V; j++)``            ``{``                ``// If vertex k is on a path``                ``// from i to j,``                ``// then make sure that the value``                ``// of reach[i][j] is 1``                ``reach[i][j] = reach[i][j] ||``                  ``(reach[i][k] && reach[k][j]);``            ``}``        ``}``    ``}` `    ``// Print the shortest distance matrix``    ``printSolution(reach);``}` `/* A utility function to print solution */``void` `printSolution(``int` `reach[][V])``{``    ``printf` `(``"Following matrix is transitive"``);``    ``printf``(``"closure of the given graph\n"``);``    ``for` `(``int` `i = 0; i < V; i++)``    ``{``        ``for` `(``int` `j = 0; j < V; j++)``        ``{``              ``/* because "i==j means same vertex"``               ``and we can reach same vertex``               ``from same vertex. So, we print 1....``               ``and we have not considered this in``               ``Floyd Warshall Algo. so we need to``               ``make this true by ourself``               ``while printing transitive closure.*/``              ``if``(i == j)``                ``printf``(``"1 "``);``              ``else``                ``printf` `(``"%d "``, reach[i][j]);``        ``}``        ``printf``(``"\n"``);``    ``}``}` `// Driver Code``int` `main()``{``    ``/* Let us create the following weighted graph``            ``10``       ``(0)------->(3)``        ``|         /|\``      ``5 |          |``        ``|          | 1``       ``\|/         |``       ``(1)------->(2)``            ``3           */``    ``int` `graph[V][V] = { {1, 1, 0, 1},``                        ``{0, 1, 1, 0},``                        ``{0, 0, 1, 1},``                        ``{0, 0, 0, 1}``                      ``};` `    ``// Print the solution``    ``transitiveClosure(graph);``    ``return` `0;``}`

## Java

 `// Program for transitive closure``// using Floyd Warshall Algorithm``import` `java.util.*;``import` `java.lang.*;``import` `java.io.*;` `class` `GraphClosure``{``    ``final` `static` `int` `V = ``4``; ``//Number of vertices in a graph` `    ``// Prints transitive closure of graph[][] using Floyd``    ``// Warshall algorithm``    ``void` `transitiveClosure(``int` `graph[][])``    ``{``        ``/* reach[][] will be the output matrix that will finally``           ``have the shortest  distances between every pair of``           ``vertices */``        ``int` `reach[][] = ``new` `int``[V][V];``        ``int`  `i, j, k;` `        ``/* Initialize the solution matrix same as input graph``           ``matrix. Or  we can say the initial values of shortest``           ``distances are based  on shortest paths considering``           ``no intermediate vertex. */``        ``for` `(i = ``0``; i < V; i++)``            ``for` `(j = ``0``; j < V; j++)``                ``reach[i][j] = graph[i][j];` `        ``/* Add all vertices one by one to the set of intermediate``           ``vertices.``          ``---> Before start of a iteration, we have reachability``               ``values for all  pairs of vertices such that the``               ``reachability values consider only the vertices in``               ``set {0, 1, 2, .. k-1} as intermediate vertices.``          ``----> After the end of a iteration, vertex no. k is``                ``added to the set of intermediate vertices and the``                ``set becomes {0, 1, 2, .. k} */``        ``for` `(k = ``0``; k < V; k++)``        ``{``            ``// Pick all vertices as source one by one``            ``for` `(i = ``0``; i < V; i++)``            ``{``                ``// Pick all vertices as destination for the``                ``// above picked source``                ``for` `(j = ``0``; j < V; j++)``                ``{``                    ``// If vertex k is on a path from i to j,``                    ``// then make sure that the value of reach[i][j] is 1``                    ``reach[i][j] = (reach[i][j]!=``0``) ||``                             ``((reach[i][k]!=``0``) && (reach[k][j]!=``0``))?``1``:``0``;``                ``}``            ``}``        ``}` `        ``// Print the shortest distance matrix``        ``printSolution(reach);``    ``}` `    ``/* A utility function to print solution */``    ``void` `printSolution(``int` `reach[][])``    ``{``        ``System.out.println(``"Following matrix is transitive closure"``+``                           ``" of the given graph"``);``        ``for` `(``int` `i = ``0``; i < V; i++)``        ``{``            ``for` `(``int` `j = ``0``; j < V; j++) {``                ``if` `( i == j)``                  ``System.out.print(``"1 "``);``                ``else``                  ``System.out.print(reach[i][j]+``" "``);``            ``}``            ``System.out.println();``        ``}``    ``}` `    ``// Driver Code``    ``public` `static` `void` `main (String[] args)``    ``{``        ``/* Let us create the following weighted graph``           ``10``        ``(0)------->(3)``        ``|         /|\``      ``5 |          |``        ``|          | 1``        ``\|/        |``        ``(1)------->(2)``           ``3           */` `        ``/* Let us create the following weighted graph` `              ``10``         ``(0)------->(3)``          ``|         /|\``        ``5 |          |``          ``|          | 1``         ``\|/         |``         ``(1)------->(2)``            ``3           */``         ``int` `graph[][] = ``new` `int``[][]{ {``1``, ``1``, ``0``, ``1``},``                                      ``{``0``, ``1``, ``1``, ``0``},``                                      ``{``0``, ``0``, ``1``, ``1``},``                                      ``{``0``, ``0``, ``0``, ``1``}``                                    ``};` `         ``// Print the solution``         ``GraphClosure g = ``new` `GraphClosure();``         ``g.transitiveClosure(graph);``    ``}``}``// This code is contributed by Aakash Hasija`

## Python

 `# Python program for transitive closure using Floyd Warshall Algorithm``#Complexity : O(V^3)` `from` `collections ``import` `defaultdict` `#Class to represent a graph``class` `Graph:` `    ``def` `__init__(``self``, vertices):``        ``self``.V ``=` `vertices` `    ``# A utility function to print the solution``    ``def` `printSolution(``self``, reach):``        ``print` `(``"Following matrix transitive closure of the given graph "``)   ``        ``for` `i ``in` `range``(``self``.V):``            ``for` `j ``in` `range``(``self``.V):``                ``if` `(i ``=``=` `j):``                  ``print` `"%7d\t"` `%` `(``1``),``                ``else``:``                  ``print` `"%7d\t"` `%``(reach[i][j]),``            ``print` `""``    ` `    ` `    ``# Prints transitive closure of graph[][] using Floyd Warshall algorithm``    ``def` `transitiveClosure(``self``,graph):``        ``'''reach[][] will be the output matrix that will finally``        ``have reachability values.``        ``Initialize the solution matrix same as input graph matrix'''``        ``reach ``=``[i[:] ``for` `i ``in` `graph]``        ``'''Add all vertices one by one to the set of intermediate``        ``vertices.``         ``---> Before start of a iteration, we have reachability value``         ``for all pairs of vertices such that the reachability values``          ``consider only the vertices in set``        ``{0, 1, 2, .. k-1} as intermediate vertices.``          ``----> After the end of an iteration, vertex no. k is``         ``added to the set of intermediate vertices and the``        ``set becomes {0, 1, 2, .. k}'''``        ``for` `k ``in` `range``(``self``.V):``            ` `            ``# Pick all vertices as source one by one``            ``for` `i ``in` `range``(``self``.V):``                ` `                ``# Pick all vertices as destination for the``                ``# above picked source``                ``for` `j ``in` `range``(``self``.V):``                    ` `                    ``# If vertex k is on a path from i to j,``                       ``# then make sure that the value of reach[i][j] is 1``                    ``reach[i][j] ``=` `reach[i][j] ``or` `(reach[i][k] ``and` `reach[k][j])` `        ``self``.printSolution(reach)``        ` `g``=` `Graph(``4``)` `graph ``=` `[[``1``, ``1``, ``0``, ``1``],``         ``[``0``, ``1``, ``1``, ``0``],``         ``[``0``, ``0``, ``1``, ``1``],``         ``[``0``, ``0``, ``0``, ``1``]]` `#Print the solution``g.transitiveClosure(graph)` `#This code is contributed by Neelam Yadav`

## C#

 `// C# Program for transitive closure``// using Floyd Warshall Algorithm``using` `System;` `class` `GFG``{``    ``static` `int` `V = 4; ``// Number of vertices in a graph` `    ``// Prints transitive closure of graph[,]``    ``// using Floyd Warshall algorithm``    ``void` `transitiveClosure(``int` `[,]graph)``    ``{``        ``/* reach[,] will be the output matrix that``        ``will finally have the shortest distances``        ``between every pair of vertices */``        ``int` `[,]reach = ``new` `int``[V, V];``        ``int` `i, j, k;` `        ``/* Initialize the solution matrix same as``        ``input graph matrix. Or we can say the``        ``initial values of shortest distances are``        ``based on shortest paths considering no``        ``intermediate vertex. */``        ``for` `(i = 0; i < V; i++)``            ``for` `(j = 0; j < V; j++)``                ``reach[i, j] = graph[i, j];` `        ``/* Add all vertices one by one to the ``        ``set of intermediate vertices.``        ``---> Before start of a iteration, we have``            ``reachability values for all pairs of``            ``vertices such that the reachability``            ``values consider only the vertices in``            ``set {0, 1, 2, .. k-1} as intermediate vertices.``        ``---> After the end of a iteration, vertex no. ``             ``k is added to the set of intermediate``             ``vertices and the set becomes {0, 1, 2, .. k} */``        ``for` `(k = 0; k < V; k++)``        ``{``            ``// Pick all vertices as source one by one``            ``for` `(i = 0; i < V; i++)``            ``{``                ``// Pick all vertices as destination``                ``// for the above picked source``                ``for` `(j = 0; j < V; j++)``                ``{``                    ``// If vertex k is on a path from i to j,``                    ``// then make sure that the value of``                    ``// reach[i,j] is 1``                    ``reach[i, j] = (reach[i, j] != 0) ||``                                 ``((reach[i, k] != 0) &&``                                  ``(reach[k, j] != 0)) ? 1 : 0;``                ``}``            ``}``        ``}` `        ``// Print the shortest distance matrix``        ``printSolution(reach);``    ``}` `    ``/* A utility function to print solution */``    ``void` `printSolution(``int` `[,]reach)``    ``{``        ``Console.WriteLine(``"Following matrix is transitive"` `+``                          ``" closure of the given graph"``);``        ``for` `(``int` `i = 0; i < V; i++)``        ``{``            ``for` `(``int` `j = 0; j < V; j++){``                ``if` `(i == j)``                  ``Console.Write(``"1 "``);``                ``else``                  ``Console.Write(reach[i, j] + ``" "``);``            ``}``            ``Console.WriteLine();``        ``}``    ``}` `    ``// Driver Code``    ``public` `static` `void` `Main (String[] args)``    ``{``        ``/* Let us create the following weighted graph``        ``10``        ``(0)------->(3)``        ``|     /|\``    ``5 |     |``        ``|     | 1``        ``\|/ |``        ``(1)------->(2)``        ``3     */` `        ``/* Let us create the following weighted graph` `            ``10``        ``(0)------->(3)``        ``|     /|\``        ``5 |     |``        ``|     | 1``        ``\|/     |``        ``(1)------->(2)``            ``3     */``        ``int` `[,]graph = ``new` `int``[,]{{1, 1, 0, 1},``                                  ``{0, 1, 1, 0},``                                  ``{0, 0, 1, 1},``                                  ``{0, 0, 0, 1}};` `        ``// Print the solution``        ``GFG g = ``new` `GFG();``        ``g.transitiveClosure(graph);``    ``}``}` `// This code is contributed by 29AjayKumar`

## Javascript

 ``
Output
```Following matrix is transitiveclosure of the given graph
1 1 1 1
0 1 1 1
0 0 1 1
0 0 0 1 ```

Time Complexity: O(V3) where V is number of vertices in the given graph.
See below post for a O(V2) solution.
Transitive Closure of a Graph using DFS
References:
Introduction to Algorithms by Clifford Stein, Thomas H. Cormen, Charles E. Leiserson, Ronald L.