Minimum cost of path between given nodes containing at most K nodes in a directed and weighted graph

Given a directed weighted graph represented by a 2-D array graph[][] of size n and 3 integers src, dst, and k representing the source point, destination point, and the available number of stops. The task is to minimize the cost of the path between two nodes containing at most K nodes in a directed and weighted graph. If there is no such route, return -1.

Examples:

Input: n=6, graph[][] = [[0, 1, 10], [1, 2, 20], [1, 3, 10], [2, 5, 30], [3, 4, 10], [4, 5, 10]], src=0, dst=5, k=2
Output: 60
Explanation:

There can be a route marked with a green arrow that takes cost =  10+10+10+10=40 using three stops. And route marked with red arrow takes cost = 10+20+30=60 using two stops. But since there can be at most 2 stops, the answer will be 60.

Input: n=3, graph[][] = [[0, 1, 10], [0, 2, 50], [1, 2, 10], src=0, dst=2, k=1
Output:  20
Explanation:

Since the k is 1, then the green-colored path can be taken with a minimum cost of 20.

Approach: Increase k by 1 because on reaching the destination, k+1 stops will be consumed. Use Breadth-first search to run while loop till the queue becomes empty. At each time pop out all the elements from the queue and decrease k by 1 and check-in their adjacent list of elements and check they are not visited before or their cost is greater than the cost of parent node + cost of that node, then mark their prices by prices[parent] + cost of that node. If k becomes 0 then break the while loop because either cost is calculated or we consumed k+1 stops. Follow the steps below to solve the problem:

• Increase the value of k by 1.
• Initialize a vector of pair adj[n] and construct the adjacency list for the graph.
• Initialize a vector prices[n] with values -1.
• Initialize a queue of pair q[].
• Push the pair {src, 0} into the queue and set the value of prices[0] as 0.
• Traverse in a while loop until the queue becomes empty and perform the following steps:
  • If k equals 0 then break.
  • Initialize the variable sz as the size of the queue.
  • Traverse in a while loop until sz is greater than 0 and perform the following tasks:
    • Initialize the variables node as the first value of the front pair of the queue and cost as the second value of the front pair of the queue.
    • Iterate over the range [0, adj[node].size()) using the variable it and perform the following tasks:
      • If prices[it.first] equals -1 or cost + it.second is less than prices[it.first] then set the value of prices[it.first] as cost + it.second and push the pair {it.first, cost + it.second} into the queue.
  • Reduce the value of k by 1.
• After performing the above steps, print the value of prices[dst] as the answer.

Below is the implementation of the above approach: 

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;

// Function to find the minimum cost
// from src to dst with at most k stops
int findCheapestCost(int n,
                     vector<vector<int> >& graph,
                     int src, int dst, int k)
{
      //if the destination cannot be reached
      if(dst > n)
      return -1;
    // Increase k by 1 Because on reaching
    // destination, k becomes k+1
    k = k + 1;

    // Making Adjacency List
    vector<pair<int, int> > adj[n];

    // U->{v, wt}
    for (auto it : graph) {
        adj[it[0]].push_back({ it[1], it[2] });
    }

    // Vector for Storing prices
    vector<int> prices(n, -1);

    // Queue for storing vertex and cost
    queue<pair<int, int> > q;

    q.push({ src, 0 });
    prices[src] = 0;

    while (!q.empty()) {

        // If all the k stops are used,
        // then break
        if (k == 0)
            break;

        int sz = q.size();
        while (sz--) {
            int node = q.front().first;
            int cost = q.front().second;
            q.pop();

            for (auto it : adj[node]) {
                if (prices[it.first] == -1
                    or cost + it.second
                           < prices[it.first]) {
                    prices[it.first] = cost + it.second;
                    q.push({ it.first, cost + it.second });
                }
            }
        }
        k--;
    }
    return prices[dst];
}

// Driver Code
int main()
{
    int n = 6;
    vector<vector<int> > graph
        = { { 0, 1, 10 }, { 1, 2, 20 }, { 2, 5, 30 }, { 1, 3, 10 }, { 3, 4, 10 }, { 4, 5, 10 } };

    int src = 0;
    int dst = 5;
    int k = 2;
    cout << findCheapestCost(n, graph, src, dst, k)
         << endl;

    return 0;
}

// Java program for the above approach
import java.util.*;

public class GFG{
  static class pair
  { 
    int first, second; 
    public pair(int first, int second)  
    { 
      this.first = first; 
      this.second = second; 
    }    
  } 
  // Function to find the minimum cost
  // from src to dst with at most k stops
  static int findCheapestCost(int n,
                              int[][] graph,
                              int src, int dst, int k)
  {
    //if the destination cannot be reached
      if(dst > n)
      return -1;
    // Increase k by 1 Because on reaching
    // destination, k becomes k+1
    k = k + 1;

    // Making Adjacency List
    Vector<pair> []adj = new Vector[n];
    for (int i = 0; i < adj.length; i++)
      adj[i] = new Vector<pair>();

    // U.{v, wt}
    for (int it[] : graph) {
      adj[it[0]].add(new pair( it[1], it[2] ));
    }

    // Vector for Storing prices
    int []prices = new int[n];
    Arrays.fill(prices, -1);

    // Queue for storing vertex and cost
    Queue<pair > q = new LinkedList<>();

    q.add(new pair( src, 0 ));
    prices[src] = 0;

    while (!q.isEmpty()) {

      // If all the k stops are used,
      // then break
      if (k == 0)
        break;

      int sz = q.size();
      while (sz-- >0) {
        int node = q.peek().first;
        int cost = q.peek().second;
        q.remove();

        for (pair it : adj[node]) {
          if (prices[it.first] == -1
              || cost + it.second
              < prices[it.first]) {
            prices[it.first] = cost + it.second;
            q.add(new pair( it.first, cost + it.second ));
          }
        }
      }
      k--;
    }
    return prices[dst];
  }

  // Driver Code
  public static void main(String[] args)
  {
    int n = 6;
    int[][] graph
      = { { 0, 1, 10 }, { 1, 2, 20 }, { 2, 5, 30 }, { 1, 3, 10 }, { 3, 4, 10 }, { 4, 5, 10 } };

    int src = 0;
    int dst = 5;
    int k = 2;
    System.out.print(findCheapestCost(n, graph, src, dst, k)
                     +"\n");

  }
}

// This code is contributed by 29AjayKumar 

# python3 program for the above approach
from collections import deque

# Function to find the minimum cost
# from src to dst with at most k stops


def findCheapestCost(n, graph, src, dst, k):

    # if the destination cannot be reached
    if dst > n:
      return -1;
    # Increase k by 1 Because on reaching
    # destination, k becomes k+1
    k = k + 1

    # Making Adjacency List
    adj = [[] for _ in range(n)]

    # U->{v, wt}
    for it in graph:
        adj[it[0]].append([it[1], it[2]])

    # Vector for Storing prices
    prices = [-1 for _ in range(n)]

    # Queue for storing vertex and cost
    q = deque()

    q.append([src, 0])
    prices[src] = 0

    while (len(q) != 0):

        # If all the k stops are used,
        # then break
        if (k == 0):
            break

        sz = len(q)
        while (True):
            sz -= 1

            pr = q.popleft()

            node = pr[0]
            cost = pr[1]

            for it in adj[node]:
                if (prices[it[0]] == -1
                        or cost + it[1]
                        < prices[it[0]]):
                    prices[it[0]] = cost + it[1]
                    q.append([it[0], cost + it[1]])

            if sz == 0:
                break
        k -= 1

    return prices[dst]

# Driver Code
if __name__ == "__main__":

    n = 6
    graph = [
            [0, 1, 10],
            [1, 2, 20],
            [2, 5, 30],
            [1, 3, 10],
            [3, 4, 10],
            [4, 5, 10]]

    src = 0
    dst = 5
    k = 2
    print(findCheapestCost(n, graph, src, dst, k))

    # This code is contributed by rakeshsahni

using System;
using System.Collections.Generic;

class Program
{
    // Function to find the minimum cost
    // from src to dst with at most k stops
    public static int findCheapestCost(int n, int[,] graph, int src, int dst, int k)
    {
        // if the destination cannot be reached
        if (dst > n)
        {
            return -1;
        }

        // Increase k by 1 Because on reaching
        // destination, k becomes k+1
        k = k + 1;

        // Making Adjacency List
        List<List<int[]>> adj = new List<List<int[]>>();
        for (int i = 0; i < n; i++)
        {
            adj.Add(new List<int[]>());
        }

        // U->{v, wt}
        for (int i = 0; i < graph.GetLength(0); i++)
        {
            adj[graph[i, 0]].Add(new int[] { graph[i, 1], graph[i, 2] });
        }

        // Vector for Storing prices
        int[] prices = new int[n];
        for (int i = 0; i < n; i++)
        {
            prices[i] = -1;
        }

        // Queue for storing vertex and cost
        Queue<int[]> q = new Queue<int[]>();
        q.Enqueue(new int[] { src, 0 });
        prices[src] = 0;

        while (q.Count != 0)
        {
            // If all the k stops are used,
            // then break
            if (k == 0)
            {
                break;
            }

            int sz = q.Count;
            while (sz-- > 0)
            {
                int[] pr = q.Dequeue();

                int node = pr[0];
                int cost = pr[1];

                foreach (int[] it in adj[node])
                {
                    if (prices[it[0]] == -1
                        || cost + it[1] < prices[it[0]])
                    {
                        prices[it[0]] = cost + it[1];
                        q.Enqueue(new int[] { it[0], cost + it[1] });
                    }
                }
            }

            k--;
        }

        return prices[dst];
    }

    public static void Main()
    {
        int n = 6;
        int[,] graph = {
            {0, 1, 10},
            {1, 2, 20},
            {2, 5, 30},
            {1, 3, 10},
            {3, 4, 10},
            {4, 5, 10}
        };

        int src = 0;
        int dst = 5;
        int k = 2;
        Console.WriteLine(findCheapestCost(n, graph, src, dst, k));
    }
}

function findCheapestCost(n, graph, src, dst, k) {
  // if the destination cannot be reached
  if (dst > n) {
    return -1;
  }

  // Increase k by 1 Because on reaching
  // destination, k becomes k+1
  k = k + 1;

  // Making Adjacency List
  let adj = Array.from({ length: n }, () => []);
  for (let i = 0; i < graph.length; i++) {
    adj[graph[i][0]].push([graph[i][1], graph[i][2]]);
  }

  // Vector for Storing prices
  let prices = Array.from({ length: n }, () => -1);

  // Queue for storing vertex and cost
  let q = [];
  q.push([src, 0]);
  prices[src] = 0;

  while (q.length != 0) {
    // If all the k stops are used,
    // then break
    if (k == 0) {
      break;
    }

    let sz = q.length;
    while (sz > 0) {
      sz -= 1;
      let pr = q.shift();
      let node = pr[0];
      let cost = pr[1];

      for (let i = 0; i < adj[node].length; i++) {
        let it = adj[node][i];
        if (prices[it[0]] == -1 || cost + it[1] < prices[it[0]]) {
          prices[it[0]] = cost + it[1];
          q.push([it[0], cost + it[1]]);
        }
      }

      if (sz == 0) {
        break;
      }
    }

    k -= 1;
  }

  return prices[dst];
}

let n = 6;
let graph = [  [0, 1, 10],
  [1, 2, 20],
  [2, 5, 30],
  [1, 3, 10],
  [3, 4, 10],
  [4, 5, 10],
];
let src = 0;
let dst = 5;
let k = 2;

console.log(findCheapestCost(n, graph, src, dst, k));

60

Time Complexity: O(n*k)
Auxiliary Space: O(n)