Number of shortest paths in an Undirected Weighted Graph

Last Updated : 05 Jul, 2021

Given a weighted undirected graph G and an integer S, the task is to print the distances of the shortest paths and the count of the number of the shortest paths for each node from a given vertex, S.

Examples:

Input: S =1, G =
Output: Shortest Paths distances are : 0 1 2 4 5 3 2 1 3
Numbers of the shortest Paths are: 1 1 1 2 3 1 1 1 2
Explanation:

The distance of the shortest paths to vertex 1 is 0 and there is only 1 such path, which is {1}.
The distance of the shortest paths to vertex 2 is 1 and there is only 1 such path, which is {1→2}.
The distance of the shortest paths to vertex 3 is 2 and there is only 1 such path, which is {1→2→3}.
The distance of the shortest paths to vertex 4 is 4 and there exist 2 such paths, which are {{1→2→3→4}, {1→2→3→6→4}}.
The distance of the shortest paths to vertex 5 is 5 and there exist 3 such paths, which are {{1→2→3→4→5}, {1→2→3→6→4→5}, {1→2→3→6→5}}.
The distance of the shortest paths to vertex 6 is 3 and there is only 1 such path, which is {1→2→3→6}.
The distance of the shortest paths to vertex 7 is 2 and there is only 1 such path, which is {1→8→7}.
The distance of the shortest paths to vertex 8 is 1 and there is only 1 such path, which is {1→8}.
The distance of the shortest paths to vertex 9 is 3 and there exist 2 such paths, which are {{1→8→9}, {1→2→3→9}}.

Approach: The given problem can be solved using the Dijkstra Algorithm. Follow the steps below to solve the problem:

Form the adjacency List of the given graph using ArrayList<ArrayList<>> and store it in a variable, say adj.
Initialize two integers, Arrays say Dist[] and Paths[] all elements as 0 to store the shortest distances of each node and count of paths with the shortest distance from the source Node, S.
Define a function, say Dijkstra() to find the shortest distances of each node and count the paths with the shortest distance:
- Initialize a min PriorityQueue say PQ and a HashSet of Strings say settled to store if the edge is visited or not.
- Assign 0 to Dist[S] and 1 to Paths[S].
- Now iterate until PQ is not empty() and perform the following operations:
  - Find the top Node of the PQ and store the Node value in a variable u.
  - Pop the top element of PQ.
  - Iterate over the ArrayList adj[u] and perform the following operations
    - Store the adjacent node in a variable say to and edge cost in a variable say cost:
    - If edge {u, to} is visited, then continue.
    - If dist[to] is greater than dist[u]+cost, then assign dist[u]+cost to dist[to] and then assign Paths[u] to Paths[to].
    - Otherwise, if Paths[to] is equal to dist[u]+cost, then increment Paths[to] by 1.
    - Now, Mark, the current edge {u, to} visited in settled.
Call the function Dijkstra().
Finally, print the Arrays dist[] and Paths[].

Below is the implementation of the above approach:

Java

// Java program for the above approach
import java.io.*;
import java.util.*;
class GFG {
  
    // Node class
    static class Node implements Comparator<Node> {
  
        // Stores the node
        public int node;
  
        // Stores the weight
        // of the edge
        public int cost;
  
        public Node() {}
  
        // Constructor
        public Node(int node, int cost)
        {
            this.node = node;
            this.cost = cost;
        }
  
        // Costume comparator
        @Override
        public int compare(Node node1, Node node2)
        {
            if (node1.cost < node2.cost)
                return -1;
            if (node1.cost > node2.cost)
                return 1;
            return 0;
        }
    }
  
    // Function to insert a node
    // in adjacency list
    static void addEdge(ArrayList<ArrayList<Node> > adj,
                        int x, int y, int w)
    {
        adj.get(x).add(new Node(y, w));
        adj.get(y).add(new Node(x, w));
    }
  
    // Auxiliary function to find shortest paths using
    // Dijekstra
    static void dijkstra(ArrayList<ArrayList<Node> > adj,
                         int src, int n, int dist[],
                         int paths[])
    {
        // Stores the distances of every node in shortest
        // order
        PriorityQueue<Node> pq
            = new PriorityQueue<Node>(n + 1, new Node());
  
        // Stores if a vertex has been visited or not
        Set<String> settled = new HashSet<String>();
  
        // Adds the source node with 0 distance to pq
        pq.add(new Node(src, 0));
  
        dist[src] = 0;
        paths[src] = 1;
  
        // While pq is not empty()
        while (!pq.isEmpty()) {
  
            // Stores the top node of pq
            int u = pq.peek().node;
  
            // Stores the distance
            // of node u from s
            int d = pq.peek().cost;
  
            // Pop the top element
            pq.poll();
  
            for (int i = 0; i < adj.get(u).size(); i++) {
                int to = adj.get(u).get(i).node;
                int cost = adj.get(u).get(i).cost;
  
                // If edge is marked
                if (settled.contains(to + " " + u))
                    continue;
  
                // If dist[to] is greater
                // than dist[u] + cost
                if (dist[to] > dist[u] + cost) {
  
                    // Add the node to to the pq
                    pq.add(new Node(to, d + cost));
  
                    // Update dist[to]
                    dist[to] = dist[u] + cost;
  
                    // Update paths[to]
                    paths[to] = paths[u];
                }
  
                // Otherwise
                else if (dist[to] == dist[u] + cost) {
                    paths[to] = (paths[to] + paths[u]);
                }
  
                // Mark the edge visited
                settled.add(to + " " + u);
            }
        }
    }
  
    // Function to find the count of shortest path and
    // distances from source node to every other node
    static void
    findShortestPaths(ArrayList<ArrayList<Node> > adj,
                      int s, int n)
    {
        // Stores the distances of a
        // node from source node
        int[] dist = new int[n + 5];
  
        // Stores the count of shortest
        // paths of a node from
        // source node
        int[] paths = new int[n + 5];
  
        for (int i = 0; i <= n; i++)
            dist[i] = Integer.MAX_VALUE;
  
        for (int i = 0; i <= n; i++)
            paths[i] = 0;
  
        // Function call to find
        // the shortest paths
        dijkstra(adj, s, n, dist, paths);
  
        System.out.print("Shortest Paths distances are : ");
        for (int i = 1; i <= n; i++) {
            System.out.print(dist[i] + " ");
        }
  
        System.out.println();
  
        System.out.print(
            "Numbers of the shortest Paths are: ");
        for (int i = 1; i <= n; i++)
            System.out.print(paths[i] + " ");
    }
  
    // Driver Code
    public static void main(String[] args)
    {
        // Input
        int N = 9;
        int M = 14;
  
        ArrayList<ArrayList<Node> > adj = new ArrayList<>();
  
        for (int i = 0; i <= N; i++) {
            adj.add(new ArrayList<Node>());
        }
  
        addEdge(adj, 1, 2, 1);
        addEdge(adj, 2, 3, 1);
        addEdge(adj, 3, 4, 2);
        addEdge(adj, 4, 5, 1);
        addEdge(adj, 5, 6, 2);
        addEdge(adj, 6, 7, 2);
        addEdge(adj, 7, 8, 1);
        addEdge(adj, 8, 1, 1);
        addEdge(adj, 2, 8, 2);
        addEdge(adj, 3, 9, 1);
        addEdge(adj, 8, 9, 2);
        addEdge(adj, 7, 9, 2);
        addEdge(adj, 3, 6, 1);
        addEdge(adj, 4, 6, 1);
  
        // Function call
        findShortestPaths(adj, 1, N);
    }
}

Output:

Shortest Paths distances are : 0 1 2 4 5 3 2 1 3 
Numbers of the shortest Paths are: 1 1 1 2 3 1 1 1 2

Time Complexity: O(M + N * log(N))
Auxiliary Space: O(M)

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