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Find all remaining vertices in Graph after marking shortest path for neighbours

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  • Difficulty Level : Basic
  • Last Updated : 05 Sep, 2022
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Given an undirected graph with V vertices in the form of an adjacency matrix, the weight of any two edges cannot be the same. For each vertex mark vertex that is connected to it via the edge of the lowest weight, the task is to return all the unmarked vertices. Return -1 if every vertex is marked.

Examples:

Input: V = 3, A[][] = {{0, 10, 50}, {10, 0, 30}, {50, 30, 0}}
Output: {2}
Explanation: 
For vertex 0 -> 1 is marked, 
for vertex 1 -> 0 is marked and 
for vertex 2 -> 1 is marked
So 2 remained unmarked.

 

Input: V = 5, A[][] = {{0, 120, 150, 115, 200}, {120, 0, 30, 40, 170}, {150, 30, 0, 50, 160}, {115, 40, 50, 0, 155}, {200, 180, 160, 155, 0}}
Output: {0, 4}

Approach: To solve the problem follow the below idea:

The idea is to simply traverse each vertex and mark the vertex having connected to current vertex using the lowest weight edge, and find all the unmarked edges.

Follow the steps to solve this problem:

  • Iterate the whole adjacency matrix using 2 loops i and j.
  • For each vertex(row) find the lowest value in the adjacency matrix and the column number of that value would be the vertex having connected to the current vertex using the lowest weight edge,
  • Mark this vertex for each vertex, using a boolean array. 
  • Print all the vertices not marked in the boolean array

Below is the implementation of this approach:

C++




// Include header file
#include <iostream>
#include <string>
#include <vector>
#include <limits>
 
// Stdc++11 code to implement the approach
class GFG
{
   
    // Function to calculate the number
    // of unmarked edges
    public:
    static std::vector<bool> markedNodesCounter(std::vector<std::vector<int>> &A, int V)
    {
       
        // Initialising visited array
        std::vector<bool> marked(V);
       
        // Loop to iterate through
        // every node
        for (int i = 0; i < V; i++)
        {
           
            // Initialising the min. distance
            // and the node closest to the
            // current node
            int minWeight = std::numeric_limits<int>::max();
            int min = V + 1;
           
            // loop to find the weights
            // from current node to every
            // other node
            for (int j = 0; j < V; j++)
            {
                if (i == j)
                {
                    continue;
                }
                if (A[i][j] < minWeight)
                {
                    minWeight = A[i][j];
                    min = j;
                }
            }
            // Marking the closest node
            marked[min] = true;
        }
        // Returning the answer
        return marked;
    }
    // Driver Code
    static void main(std::vector<std::string> &args)
    {
        int V = 3;
        std::vector<std::vector<int>> A{{0, 10, 50}, {10, 0, 30}, {50, 30, 0}};
        // Fetching result
        std::vector<bool> marked = GFG::markedNodesCounter(A, V);
        // Printing result
        bool flag = false;
        for (int i = 0; i < V; i++)
        {
            if (!marked[i])
            {
                std::cout << std::to_string(i) + " ";
                flag = true;
            }
        }
        if (!flag)
        {
            std::cout << "-1" << std::endl;
        }
        else
        {
            std::cout << std::endl;
        }
    }
};
int main(int argc, char **argv){
    std::vector<std::string> parameter(argv + 1, argv + argc);
    GFG::main(parameter);
    return 0;
};
 
// This code is contributed by aadityapburujwale.

Java




// Java code to implement the approach
class GFG {
 
    // Function to calculate the number
    // of unmarked edges
    static boolean[] markedNodesCounter(int[][] A, int V)
    {
 
        // Initialising visited array
        boolean marked[] = new boolean[V];
 
        // Loop to iterate through
        // every node
        for (int i = 0; i < V; i++) {
 
            // Initialising the min. distance
            // and the node closest to the
            // current node
            int minWeight = Integer.MAX_VALUE;
            int min = V + 1;
 
            // loop to find the weights
            // from current node to every
            // other node
            for (int j = 0; j < V; j++) {
                if (i == j)
                    continue;
                if (A[i][j] < minWeight) {
                    minWeight = A[i][j];
                    min = j;
                }
            }
 
            // Marking the closest node
            marked[min] = true;
        }
 
        // Returning the answer
        return marked;
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        int V = 3;
        int A[][] = { { 0, 10, 50 },
                      { 10, 0, 30 },
                      { 50, 30, 0 } };
 
        // Fetching result
        boolean marked[] = markedNodesCounter(A, V);
 
        // Printing result
        boolean flag = false;
        for (int i = 0; i < V; i++) {
            if (!marked[i]) {
                System.out.print(i + " ");
                flag = true;
            }
        }
        if (!flag)
            System.out.println("-1");
        else
            System.out.println();
    }
}

Python3




# Python code to implement the approach
 
# Function to calculate the number
# of unmarked edges
def markedNodesCounter(A, V):
   
    # Initialising visited array
    marked = [False] * V
 
    # Loop to iterate through
    # every node
    for i in range(V):
        # Initialising the min. distance
        # and the node closest to the
        # current node
        minWeight = float('inf')
        min = V + 1
 
        # loop to find the weights
        # from current node to every
        # other node
        for j in range(V):
            if i == j:
                continue
            if A[i][j] < minWeight:
                minWeight = A[i][j]
                min = j
 
        # Marking the closest node
        marked[min] = True
 
    # Returning the answer
    return marked
 
# Driver Code
if __name__ == '__main__':
    V = 3
    A = [[0, 10, 50], [10, 0, 30], [50, 30, 0]]
 
    # Fetching result
    marked = markedNodesCounter(A, V)
 
    # Printing result
    flag = False
    for i in range(V):
        if not marked[i]:
            print(i, end=" ")
            flag = True
 
    if not flag:
        print(-1)
    else:
        print()
 
       # This code is contributed by tapeshdua420.

C#




// Include namespace system
using System;
 
// C# code to implement the approach
public class GFG
{
    // Function to calculate the number
    // of unmarked edges
    public static bool[] markedNodesCounter(int[,] A, int V)
    {
        // Initialising visited array
        bool[] marked = new bool[V];
        // Loop to iterate through
        // every node
        for (int i = 0; i < V; i++)
        {
            // Initialising the min. distance
            // and the node closest to the
            // current node
            var minWeight = int.MaxValue;
            var min = V + 1;
            // loop to find the weights
            // from current node to every
            // other node
            for (int j = 0; j < V; j++)
            {
                if (i == j)
                {
                    continue;
                }
                if (A[i,j] < minWeight)
                {
                    minWeight = A[i,j];
                    min = j;
                }
            }
            // Marking the closest node
            marked[min] = true;
        }
        // Returning the answer
        return marked;
    }
    // Driver Code
    public static void Main(String[] args)
    {
        var V = 3;
        int[,] A = {{0, 10, 50}, {10, 0, 30}, {50, 30, 0}};
        // Fetching result
        bool[] marked = GFG.markedNodesCounter(A, V);
        // Printing result
        var flag = false;
        for (int i = 0; i < V; i++)
        {
            if (!marked[i])
            {
                Console.Write(i.ToString() + " ");
                flag = true;
            }
        }
        if (!flag)
        {
            Console.WriteLine("-1");
        }
        else
        {
            Console.WriteLine();
        }
    }
}
 
// This code is contributed by aadityapburujwale.

Javascript




<script>
 
    // Function to calculate the number
    // of unmarked edges
    function markedNodesCounter(A, V){
        // Initialising visited array
        const marked = new Array(3).fill(false);
         
        // Loop to iterate through
        // every node
        for(let i = 0; i < V; i++)
        {
         
            // Initialising the min. distance
            // and the node closest to the
            // current node
            let minWeight = Number.MAX_VALUE;
            let min = V + 1
             
            // loop to find the weights
            // from current node to every
            // other node
            for(let j = 0; j < V; j++)
            {
                if(i == j){
                    continue;
                }
                if(A[i][j] < minWeight){
                    minWeight = A[i][j];
                    min = j;
                }
            }
             
            // Marking the closest node
            marked[min] = true;
        }
        // Returning the answer
        return marked;
    }
 
    // Driver code
    let V = 3;
    var A = [ [ 0, 10, 50 ],
              [ 10, 0, 30 ],
              [ 50, 30, 0 ] ];
       
    // Fetching result
    var marked = markedNodesCounter(A, V);
     
    // Printing result
    var flag = false;
    for(let i=0;i<V;i++){
        if(!marked[i]){
            document.write(i+" ");
            flag = true;
        }
    }
    if(!flag){
        document.write("-1");
    }
    else{
        document.write("<br>");
    }
     
</script>

Output

2 

Time Complexity: O(V2), for using two nested loops from 0 to V.
Auxiliary Space: O(V), for creating an additional array marked of size V.


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