Number of shortest paths in an Undirected Weighted Graph

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:

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

C#

// C# program for the above approach

using System;
using System.Collections.Generic;

    // Node class
     class Node : IComparable<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

        public  int CompareTo(Node node2)
        {
            if (this.cost < node2.cost)
                return -1;
            if (this.cost > node2.cost)
                return 1;
            return 0;
        }
    }

class GFG {



    // Function to insert a node
    // in adjacency list
    static void addEdge(List<List<Node> > adj,
                        int x, int y, int w)
    {
        adj[x].Add(new Node(y, w));
        adj[y].Add(new Node(x, w));
    }

    // Auxiliary function to find shortest paths using
    // Dijekstra
    static void dijkstra(List<List<Node> > adj,
                         int src, int n, int[] dist,
                         int[] paths)
    {
        // Stores the distances of every node in shortest
        // order
        List<Node> pq
            = new List<Node>();

        for (int i = 0; i <= n; i++)
            pq.Add(new Node());

        // Stores if a vertex has been visited or not
        HashSet<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.Count != 0) {

            pq.Sort();

            // Stores the top node of pq
            int u = pq[0].node;

            // Stores the distance
            // of node u from s
            int d = pq[0].cost;

            // Pop the top element
            pq.RemoveAt(0);

            for (int i = 0; i < adj[u].Count; i++) {
                int to = adj[u][i].node;
                int cost = adj[u][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(List<List<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] = Int32.MaxValue;

        for (int i = 0; i <= n; i++)
            paths[i] = 0;

        // Function call to find
        // the shortest paths
        dijkstra(adj, s, n, dist, paths);

        Console.Write("Shortest Paths distances are : ");
        for (int i = 1; i <= n; i++) {
            Console.Write(dist[i] + " ");
        }

        Console.WriteLine();

        Console.Write(
            "Numbers of the shortest Paths are: ");
        for (int i = 1; i <= n; i++)
            Console.Write(paths[i] + " ");
    }

    // Driver Code
    public static void Main(string[] args)
    {
        // Input
        int N = 9;

        List<List<Node> > adj = new List<List<Node>>();

        for (int i = 0; i <= N; i++) {
            adj.Add(new List<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);
    }
}


// This code is contributed by phasing17

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)