# Implementation of Johnson’s algorithm for all-pairs shortest paths

• Difficulty Level : Expert
• Last Updated : 25 Sep, 2021

Johnson’s algorithm finds the shortest paths between all pairs of vertices in a weighted directed graph. It allows some of the edge weights to be negative numbers, but no negative-weight cycles may exist. It uses the Bellman-Ford algorithm to re-weight the original graph, removing all negative weights. Dijkstra’s algorithm is applied on the re-weighted graph to compute the shortest path between all pairs of vertices.

Algorithm Description

Using Dijkstra’s algorithm, the shortest paths between all pairs of vertices in O(V2logV) can be found. However, Dijkstra does not work with negative weights. To avoid this problem, Johnson’s algorithm uses a technique called reweighting.

Reweighting is a process by which each edge weight is changed to satisfy two properties-

• For all pairs of vertices u, v in the graph, if the shortest path exists between those vertices before reweighting, it must also be the shortest path between those vertices after reweighting.
• For all edges, (u, v), in the graph, they must have a non-negative weight (u, v).

Johnson’s algorithm uses Bellman-Ford to reweight the edges. Bellman-Ford is also able to detect negative weight cycles if present in the original graph.

Graph Representation

Adjacency List is modified a bit to represent the graph. For each source vertex s, each of its neighboring vertices has two properties associated with them:

1. Destination
2. Weight

Consider the graph –

Source vertex 0 has one neighboring vertex, one whose destination is 2 and weight is -2. Each neighboring vertex is encapsulated using a static Neighbor class.

## Java

 private static class Neighbour {    int destination;    int weight;     Neighbour(int destination, int weight)    {        this.destination = destination;        this.weight = weight;    }}

#### Pseudocode

Follow the steps below to solve the problem:

• Add a new node q to the graph, connected by zero-weight edges to all the other nodes.
• Use the Bellman-Ford algorithm, starting from the new vertex q, to find for each vertex v the minimum weight h(v) of a path from q to v. If this step detects a negative cycle, the algorithm is terminated.
• Reweight the edges of the original graph using the values computed by the Bellman-Ford algorithm: an edge from u to v, having length w(u, v) reweighted to w(u, v) + h(u) − h(v).
• Remove q and apply Dijkstra’s algorithm to find the shortest paths from each node s to every other vertex in the reweighted graph.
• Compute the distance in the original graph by adding h(v) − h(u) to the distance returned by Dijkstra’s algorithm.

Below is the implementation of the above approach:

## Java

 // Java program for the above approachimport java.util.ArrayList;import java.util.Arrays; public class Graph {    private static class Neighbour {        int destination;        int weight;         Neighbour(int destination, int weight)        {            this.destination = destination;            this.weight = weight;        }    }     private int vertices;    private final ArrayList >        adjacencyList;     // On using the below constructor,    // edges must be added manually    // to the graph using addEdge()    public Graph(int vertices)    {        this.vertices = vertices;         adjacencyList = new ArrayList<>(vertices);        for (int i = 0; i < vertices; i++)            adjacencyList.add(new ArrayList<>());    }     // On using the below constructor,    // edges will be added automatically    // to the graph using the adjacency matrix    public Graph(int vertices, int[][] adjacencyMatrix)    {        this(vertices);         for (int i = 0; i < vertices; i++) {            for (int j = 0; j < vertices; j++) {                if (adjacencyMatrix[i][j] != 0)                    addEdge(i, j, adjacencyMatrix[i][j]);            }        }    }     public void addEdge(int source, int destination,                        int weight)    {        adjacencyList.get(source).add(            new Neighbour(destination, weight));    }     // Time complexity of this    // implementation of dijkstra is O(V^2).    public int[] dijkstra(int source)    {        boolean[] isVisited = new boolean[vertices];        int[] distance = new int[vertices];         Arrays.fill(distance, Integer.MAX_VALUE);        distance = 0;         for (int vertex = 0; vertex < vertices; vertex++) {            int minDistanceVertex = findMinDistanceVertex(                distance, isVisited);            isVisited[minDistanceVertex] = true;             for (Neighbour neighbour :                 adjacencyList.get(minDistanceVertex)) {                int destination = neighbour.destination;                int weight = neighbour.weight;                 if (!isVisited[destination]                    && distance[minDistanceVertex] + weight                           < distance[destination])                    distance[destination]                        = distance[minDistanceVertex]                          + weight;            }        }         return distance;    }     // Method used by `int[] dijkstra(int)`    private int findMinDistanceVertex(int[] distance,                                      boolean[] isVisited)    {        int minIndex = -1,            minDistance = Integer.MAX_VALUE;         for (int vertex = 0; vertex < vertices; vertex++) {            if (!isVisited[vertex]                && distance[vertex] <= minDistance) {                minDistance = distance[vertex];                minIndex = vertex;            }        }         return minIndex;    }     // Returns null if    // negative weight cycle is detected    public int[] bellmanford(int source)    {        int[] distance = new int[vertices];         Arrays.fill(distance, Integer.MAX_VALUE);        distance = 0;         for (int i = 0; i < vertices - 1; i++) {            for (int currentVertex = 0;                 currentVertex < vertices;                 currentVertex++) {                for (Neighbour neighbour :                     adjacencyList.get(currentVertex)) {                    if (distance[currentVertex]                            != Integer.MAX_VALUE                        && distance[currentVertex]                                   + neighbour.weight                               < distance                                     [neighbour                                          .destination]) {                        distance[neighbour.destination]                            = distance[currentVertex]                              + neighbour.weight;                    }                }            }        }         for (int currentVertex = 0;             currentVertex < vertices; currentVertex++) {            for (Neighbour neighbour :                 adjacencyList.get(currentVertex)) {                if (distance[currentVertex]                        != Integer.MAX_VALUE                    && distance[currentVertex]                               + neighbour.weight                           < distance[neighbour                                          .destination])                    return null;            }        }         return distance;    }     // Returns null if negative    // weight cycle is detected    public int[][] johnsons()    {        // Add a new vertex q to the original graph,        // connected by zero-weight edges to        // all the other vertices of the graph         this.vertices++;        adjacencyList.add(new ArrayList<>());        for (int i = 0; i < vertices - 1; i++)            adjacencyList.get(vertices - 1)                .add(new Neighbour(i, 0));         // Use bellman ford with the new vertex q        // as source, to find for each vertex v        // the minimum weight h(v) of a path        // from q to v.        // If this step detects a negative cycle,        // the algorithm is terminated.         int[] h = bellmanford(vertices - 1);        if (h == null)            return null;         // Re-weight the edges of the original graph using the        // values computed by the Bellman-Ford algorithm.        // w'(u, v) = w(u, v) + h(u) - h(v).         for (int u = 0; u < vertices; u++) {            ArrayList neighbours                = adjacencyList.get(u);             for (Neighbour neighbour : neighbours) {                int v = neighbour.destination;                int w = neighbour.weight;                 // new weight                neighbour.weight = w + h[u] - h[v];            }        }         // Step 4: Remove edge q and apply Dijkstra        // from each node s to every other vertex        // in the re-weighted graph         adjacencyList.remove(vertices - 1);        vertices--;         int[][] distances = new int[vertices][];         for (int s = 0; s < vertices; s++)            distances[s] = dijkstra(s);         // Compute the distance in the original graph        // by adding h[v] - h[u] to the        // distance returned by dijkstra         for (int u = 0; u < vertices; u++) {            for (int v = 0; v < vertices; v++) {                 // If no edge exist, continue                if (distances[u][v] == Integer.MAX_VALUE)                    continue;                 distances[u][v] += (h[v] - h[u]);            }        }         return distances;    }     // Driver Code    public static void main(String[] args)    {        final int vertices = 4;        final int[][] matrix = { { 0, 0, -2, 0 },                                 { 4, 0, 3, 0 },                                 { 0, 0, 0, 2 },                                 { 0, -1, 0, 0 } };         // Initialization        Graph graph = new Graph(vertices, matrix);         // Function Call        int[][] distances = graph.johnsons();         if (distances == null) {            System.out.println(                "Negative weight cycle detected.");            return;        }         // The code fragment below outputs        // an formatted distance matrix.        // Its first row and first        // column represent vertices        System.out.println("Distance matrix:");         System.out.print("   \t");        for (int i = 0; i < vertices; i++)            System.out.printf("%3d\t", i);         for (int i = 0; i < vertices; i++) {            System.out.println();            System.out.printf("%3d\t", i);            for (int j = 0; j < vertices; j++) {                if (distances[i][j] == Integer.MAX_VALUE)                    System.out.print(" X\t");                else                    System.out.printf("%3d\t",                                      distances[i][j]);            }        }    }}

Output

Distance matrix:
0      1      2      3
0      0     -1     -2      0
1      4      0      2      4
2      5      1      0      2
3      3     -1      1      0

Time Complexity: O(V2log V + VE), The time complexity of Johnson’s algorithm becomes the same as Floyd Warshall when the graphs are complete (For a complete graph E = O(V2). But for sparse graphs, the algorithm performs much better than Floyd Warshall.
Auxiliary Space: O(V*V)

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