Nodes with prime degree in an undirected Graph

Given an undirected graph with N vertices and M edges, the task is to print all the nodes of the given graph whose degree is a Prime Number.

Examples:

Input: N = 4, arr[][] = { { 1, 2 }, { 1, 3 }, { 1, 4 }, { 2, 3 }, { 2, 4 }, { 3, 4 } }
Output: 1 2 3 4
Explanation:
Below is the graph for the above information:

The degree of the node as per above graph is:
Node -> Degree
1 -> 3
2 -> 3
3 -> 3
4 -> 3
Hence, the nodes with prime degree are 1 2 3 4

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

Approach:



  1. Use Sieve of Eratosthenes to calculate the prime numbers upto 105.
  2. For each vertex, the degree can be calculated by the length of Adjacency List of the given graph at corresponding vertex.
  3. Print those vertex of the given graph whose degree is a Prime Number.

Below is the implementation of the above approach:

C++

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// C++ implementation of the approach
  
#include <bits/stdc++.h>
using namespace std;
  
int n = 10005;
  
// To store Prime Numbers
vector<bool> Prime(n + 1, true);
  
// Function to find the prime numbers
// till 10^5
void SieveOfEratosthenes()
{
  
    int i, j;
    Prime[0] = Prime[1] = false;
    for (i = 2; i * i <= 10005; i++) {
  
        // Traverse all multiple of i
        // and make it false
        if (Prime[i]) {
  
            for (j = 2 * i; j < 10005; j += i) {
                Prime[j] = false;
            }
        }
    }
}
  
// Function to print the nodes having
// prime degree
void primeDegreeNodes(int N, int M,
                      int edges[][2])
{
    // To store Adjacency List of
    // a Graph
    vector<int> Adj[N + 1];
  
    // Make Adjacency List
    for (int i = 0; i < M; i++) {
        int x = edges[i][0];
        int y = edges[i][1];
  
        Adj[x].push_back(y);
        Adj[y].push_back(x);
    }
  
    // To precompute prime numbers
    // till 10^5
    SieveOfEratosthenes();
  
    // Traverse each vertex
    for (int i = 1; i <= N; i++) {
  
        // Find size of Adjacency List
        int x = Adj[i].size();
  
        // If length of Adj[i] is Prime
        // then print it
        if (Prime[x])
            cout << i << ' ';
    }
}
  
// Driver code
int main()
{
    // Vertices and Edges
    int N = 4, M = 6;
  
    // Edges
    int edges[M][2] = { { 1, 2 }, { 1, 3 }, 
                        { 1, 4 }, { 2, 3 }, 
                        { 2, 4 }, { 3, 4 } };
  
    // Function Call
    primeDegreeNodes(N, M, edges);
  
    return 0;
}

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Java

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// Java implementation of the approach
import java.util.*;
class GFG{
  
static int n = 10005;
  
// To store Prime Numbers
static boolean []Prime = new boolean[n + 1];
  
// Function to find the prime numbers
// till 10^5
static void SieveOfEratosthenes()
{
    int i, j;
    Prime[0] = Prime[1] = false;
    for (i = 2; i * i <= 10005; i++) 
    {
  
        // Traverse all multiple of i
        // and make it false
        if (Prime[i])
        {
            for (j = 2 * i; j < 10005; j += i) 
            {
                Prime[j] = false;
            }
        }
    }
}
  
// Function to print the nodes having
// prime degree
static void primeDegreeNodes(int N, int M,
                              int edges[][])
{
    // To store Adjacency List of
    // a Graph
    Vector<Integer> []Adj = new Vector[N + 1];
    for(int i = 0; i < Adj.length; i++)
        Adj[i] = new Vector<Integer>();
  
    // Make Adjacency List
    for (int i = 0; i < M; i++) 
    {
        int x = edges[i][0];
        int y = edges[i][1];
  
        Adj[x].add(y);
        Adj[y].add(x);
    }
  
    // To precompute prime numbers
    // till 10^5
    SieveOfEratosthenes();
  
    // Traverse each vertex
    for (int i = 1; i <= N; i++) 
    {
  
        // Find size of Adjacency List
        int x = Adj[i].size();
  
        // If length of Adj[i] is Prime
        // then print it
        if (Prime[x])
            System.out.print(i + " ");
    }
}
  
// Driver code
public static void main(String[] args)
{
    // Vertices and Edges
    int N = 4, M = 6;
  
    // Edges
    int edges[][] = { { 1, 2 }, { 1, 3 }, 
                      { 1, 4 }, { 2, 3 }, 
                      { 2, 4 }, { 3, 4 } };
    Arrays.fill(Prime, true);
      
    // Function Call
    primeDegreeNodes(N, M, edges);
}
}
  
// This code is contributed by sapnasingh4991

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C#

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// C# implementation of the approach
using System;
using System.Collections.Generic;
  
class GFG{
  
static int n = 10005;
  
// To store Prime Numbers
static bool []Prime = new bool[n + 1];
  
// Function to find the prime numbers
// till 10^5
static void SieveOfEratosthenes()
{
    int i, j;
    Prime[0] = Prime[1] = false;
    for(i = 2; i * i <= 10005; i++) 
    {
         
       // Traverse all multiple of i
       // and make it false
       if (Prime[i])
       {
           for(j = 2 * i; j < 10005; j += i) 
           {
              Prime[j] = false;
           }
       }
    }
}
  
// Function to print the nodes having
// prime degree
static void primeDegreeNodes(int N, int M,
                             int [,]edges)
{
      
    // To store Adjacency List of
    // a Graph
    List<int> []Adj = new List<int>[N + 1];
    for(int i = 0; i < Adj.Length; i++)
       Adj[i] = new List<int>();
  
    // Make Adjacency List
    for(int i = 0; i < M; i++) 
    {
       int x = edges[i, 0];
       int y = edges[i, 1];
         
       Adj[x].Add(y);
       Adj[y].Add(x);
    }
      
    // To precompute prime numbers
    // till 10^5
    SieveOfEratosthenes();
  
    // Traverse each vertex
    for(int i = 1; i <= N; i++) 
    {
          
       // Find size of Adjacency List
       int x = Adj[i].Count;
         
       // If length of Adj[i] is Prime
       // then print it
       if (Prime[x])
           Console.Write(i + " ");
    }
}
  
// Driver code
public static void Main(String[] args)
{
      
    // Vertices and Edges
    int N = 4, M = 6;
  
    // Edges
    int [,]edges = { { 1, 2 }, { 1, 3 }, 
                     { 1, 4 }, { 2, 3 }, 
                     { 2, 4 }, { 3, 4 } };
                       
    for(int i = 0; i < Prime.Length; i++)
       Prime[i] = true;
      
    // Function Call
    primeDegreeNodes(N, M, edges);
}
}
  
// This code is contributed by 29AjayKumar

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Output:

1 2 3 4

Time Complexity: O(N + M), where N is the number of vertices and M is the number of edges.

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