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Minimize count of connections required to be rearranged to make all the computers connected
  • Difficulty Level : Expert
  • Last Updated : 05 Apr, 2021

Given an integer N, denoting the number of computers connected by cables forming a network and a 2D array connections[][], with each row (i, j) representing a connection between ith and jth computer, the task is to connect all the computers either directly or indirectly by removing any of the given connections and connecting two disconnected computers. 
If it’s not possible to connect all the computers, print -1
Otherwise, print the minimum number of such operations required.

Examples:

Input: N = 4, connections[][] = {{0, 1}, {0, 2}, {1, 2}}
Output: 1
Explanation: Remove the connection between computers 1 and 2 and connect the computers 1 and 3.

Input: N = 5, connections[][] = {{0, 1}, {0, 2}, {3, 4}, {2, 3}}
Output: 0

Approach: The idea is to use a concept similar to that of Minimum Spanning Tree, as in a Graph with N nodes, only N – 1 edges are required to make all the nodes connected.



Redundant edges = Total edges – [(Number of Nodes – 1) – (Number of components – 1)]

If Redundant edges > (Number of components – 1) : It is clear that there are enough wires that can be used to connect disconnected computers. Otherwise, print -1.

Follow the steps below to solve the problem:

  1. Initialize an unordered map, say adj to store the adjacency list from the given information about edges.
  2. Initialize a vector of boolean datatype, say visited, to store whether a node is visited or not.
  3. Generate the adjacency list and also calculate the number of edges.
  4. Initialize a variable, say components, to store the count of connected components.
  5. Traverse the nodes of the graph using DFS to count the number of connected components and store it in the variable components.
  6. Initialize a variable, say redundant, and store the number of redundant edges using the above formula.
  7. If redundant edges > components – 1, then the minimum number of required operations is equal to components – 1. Otherwise, print -1.

Below is the implementation of the above approach:

C++




// C++ program for the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to visit the nodes of a graph
void DFS(unordered_map<int, vector<int> >& adj,
         int node, vector<bool>& visited)
{
    // If current node is already visited
    if (visited[node])
        return;
 
    // If current node is not visited
    visited[node] = true;
 
    // Recurse for neighbouring nodes
    for (auto x : adj[node]) {
 
        // If the node is not vistied
        if (visited[x] == false)
            DFS(adj, x, visited);
    }
}
 
// Utility function to check if it is
// possible to connect all computers or not
int makeConnectedUtil(int N,
                      int connections[][2],
                      int M)
{
    // Stores whether a
    // node is visited or not
    vector<bool> visited(N, false);
 
    // Build the adjacency list
    unordered_map<int, vector<int> > adj;
 
    // Stores count of edges
    int edges = 0;
 
    // Building adjaceny list
    // from the given edges
    for (int i = 0; i < M; ++i) {
 
        // Add edges
        adj[connections[i][0]].push_back(
            connections[i][1]);
        adj[connections[i][1]].push_back(
            connections[i][0]);
 
        // Increment count of edges
        edges += 1;
    }
 
    // Stores count of components
    int components = 0;
 
    for (int i = 0; i < N; ++i) {
 
        // If node is not visited
        if (visited[i] == false) {
 
            // Increment components
            components += 1;
 
            // Perform DFS
            DFS(adj, i, visited);
        }
    }
 
    // At least N - 1 edges are required
    if (edges < N - 1)
        return -1;
 
    // Count redundant edges
    int redundant = edges - ((N - 1)
                             - (components - 1));
 
    // Check if components can be
    // rearranged using redundant edges
    if (redundant >= (components - 1))
        return components - 1;
 
    return -1;
}
 
// Function to check if it is possible
// to connect all the computers or not
void makeConnected(int N, int connections[][2], int M)
{
    // Stores counmt of minimum
    // operations required
    int minOps = makeConnectedUtil(
        N, connections, M);
 
    // Print the minimum number
    // of operations required
    cout << minOps;
}
 
// Driver Code
int main()
{
    // Given number of computers
    int N = 4;
 
    // Given set of connections
    int connections[][2] = {
        { 0, 1 }, { 0, 2 }, { 1, 2 }
    };
    int M = sizeof(connections)
            / sizeof(connections[0]);
 
    // Function call to check if it is
    // possible to connect all computers or not
    makeConnected(N, connections, M);
 
    return 0;
}

Java




// Java program for the above approach
import java.io.*;
import java.util.*;
 
class GFG {
 
    // Function to visit the nodes of a graph
    public static void DFS(HashMap<Integer, ArrayList<Integer> > adj,
                           int node, boolean visited[])
    {
        // If current node is already visited
        if (visited[node])
            return;
 
        // If current node is not visited
        visited[node] = true;
 
        // Recurse for neighbouring nodes
        for (int x : adj.get(node)) {
 
            // If the node is not vistied
            if (visited[x] == false)
                DFS(adj, x, visited);
        }
    }
 
    // Utility function to check if it is
    // possible to connect all computers or not
    public static int
    makeConnectedUtil(int N, int connections[][], int M)
    {
        // Stores whether a
        // node is visited or not
        boolean visited[] = new boolean[N];
 
        // Build the adjacency list
        HashMap<Integer, ArrayList<Integer> > adj
            = new HashMap<>();
 
        // Initialize the adjacency list
        for (int i = 0; i < N; i++) {
            adj.put(i, new ArrayList<Integer>());
        }
 
        // Stores count of edges
        int edges = 0;
 
        // Building adjaceny list
        // from the given edges
        for (int i = 0; i < M; ++i) {
 
            // Get neighbours list
            ArrayList<Integer> l1
                = adj.get(connections[i][0]);
            ArrayList<Integer> l2
                = adj.get(connections[i][0]);
 
            // Add edges
            l1.add(connections[i][1]);
            l2.add(connections[i][0]);
 
            // Increment count of edges
            edges += 1;
        }
 
        // Stores count of components
        int components = 0;
 
        for (int i = 0; i < N; ++i) {
 
            // If node is not visited
            if (visited[i] == false) {
 
                // Increment components
                components += 1;
 
                // Perform DFS
                DFS(adj, i, visited);
            }
        }
 
        // At least N - 1 edges are required
        if (edges < N - 1)
            return -1;
 
        // Count redundant edges
        int redundant
            = edges - ((N - 1) - (components - 1));
 
        // Check if components can be
        // rearranged using redundant edges
        if (redundant >= (components - 1))
            return components - 1;
 
        return -1;
    }
 
    // Function to check if it is possible
    // to connect all the computers or not
    public static void
    makeConnected(int N, int connections[][], int M)
    {
        // Stores counmt of minimum
        // operations required
        int minOps = makeConnectedUtil(N, connections, M);
 
        // Print the minimum number
        // of operations required
        System.out.println(minOps);
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        // Given number of computers
        int N = 4;
 
        // Given set of connections
        int connections[][]
            = { { 0, 1 }, { 0, 2 }, { 1, 2 } };
        int M = connections.length;
 
        // Function call to check if it is
        // possible to connect all computers or not
        makeConnected(N, connections, M);
    }
}
 
// This code is contributed by kingash.

Python3




# Python3 code for the above approach
 
# Function to visit the nodes of a graph
def DFS(adj, node, visited):
     
    # If current node is already visited
    if (visited[node]):
        return
 
    # If current node is not visited
    visited[node] = True
 
    # Recurse for neighbouring nodes
    if(node in adj):
        for x in adj[node]:
             
            # If the node is not vistied
            if (visited[x] == False):
              DFS(adj, x, visited)
 
# Utility function to check if it is
# possible to connect all computers or not
def makeConnectedUtil(N, connections, M):
     
    # Stores whether a
    # node is visited or not
    visited = [False for i in range(N)]
 
    # Build the adjacency list
    adj = {}
     
    # Stores count of edges
    edges = 0
 
    # Building adjaceny list
    # from the given edges
    for i in range(M):
         
        # Add edges
        if (connections[i][0] in adj):
            adj[connections[i][0]].append(
                connections[i][1])
        else:
            adj[connections[i][0]] = []
        if (connections[i][1] in adj):
            adj[connections[i][1]].append(
                connections[i][0])
        else:
            adj[connections[i][1]] = []
 
        # Increment count of edges
        edges += 1
 
    # Stores count of components
    components = 0
 
    for i in range(N):
         
        # If node is not visited
        if (visited[i] == False):
             
            # Increment components
            components += 1
 
            # Perform DFS
            DFS(adj, i, visited)
 
    # At least N - 1 edges are required
    if (edges < N - 1):
        return -1
 
    # Count redundant edges
    redundant = edges - ((N - 1) - (components - 1))
 
    # Check if components can be
    # rearranged using redundant edges
    if (redundant >= (components - 1)):
        return components - 1
 
    return -1
 
# Function to check if it is possible
# to connect all the computers or not
def makeConnected(N, connections, M):
     
    # Stores counmt of minimum
    # operations required
    minOps = makeConnectedUtil(N, connections, M)
 
    # Print the minimum number
    # of operations required
    print(minOps)
 
# Driver Code
if __name__ == '__main__':
     
    # Given number of computers
    N = 4
 
    # Given set of connections
    connections = [ [ 0, 1 ], [ 0, 2 ], [ 1, 2 ] ]
    M = len(connections)
 
    # Function call to check if it is
    # possible to connect all computers or not
    makeConnected(N, connections, M)
     
# This code is contributed by ipg2016107
Output: 
1

 

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

 

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