Data Modelling & AI Data Structure & Algorithm

Find two disjoint good sets of vertices in a given graph

23 June 2025

0

Given an undirected unweighted graph with N vertices and M edges. The task is to find two disjoint good sets of vertices. A set X is called good if for every edge UV in the graph at least one of the endpoint belongs to X(i.e, U or V or both U and V belong to X).
If it is not possible to make such sets then print -1.

Examples:

Input :

Output : {1 3 4 5} ,{2 6}
One disjoint good set contains vertices {1, 3, 4, 5} and other contains {2, 6}.

Input :

Output : -1

Approach:
One of the observations is that there is no edge UV that U and V are in the same set. The two good sets form a bipartition of the graph, so the graph has to be bipartite. And being bipartite is also sufficient. Read about bipartition here.

Below is the implementation of the above approach :

C++

// C++ program to find two disjoint
// good sets of vertices in a given graph
#include <bits/stdc++.h>
using namespace std;
#define N 100005
 
// For the graph
vector<int> gr[N], dis[2];
bool vis[N];
int colour[N];
bool bip;
 
// Function to add edge
void Add_edge(int x, int y)
{
    gr[x].push_back(y);
    gr[y].push_back(x);
}
 
// Bipartite function
void dfs(int x, int col)
{
    vis[x] = true;
    colour[x] = col;
 
    // Check for child vertices
    for (auto i : gr[x]) {
 
        // If it is not visited
        if (!vis[i])
            dfs(i, col ^ 1);
 
        // If it is already visited
        else if (colour[i] == col)
            bip = false;
    }
}
 
// Function to find two disjoint
// good sets of vertices in a given graph
void goodsets(int n)
{
    // Initially assume that graph is bipartite
    bip = true;
 
    // For every unvisited vertex call dfs
    for (int i = 1; i <= n; i++)
        if (!vis[i])
            dfs(i, 0);
 
    // If graph is not bipartite
    if (!bip)
        cout << -1;
    else {
 
        // Differentiate two sets
        for (int i = 1; i <= n; i++)
            dis[colour[i]].push_back(i);
 
        // Print vertices belongs to both sets
 
        for (int i = 0; i < 2; i++) {
 
            for (int j = 0; j < dis[i].size(); j++)
                cout << dis[i][j] << " ";
            cout << endl;
        }
    }
}
 
// Driver code
int main()
{
    int n = 6, m = 4;
 
    // Add edges
    Add_edge(1, 2);
    Add_edge(2, 3);
    Add_edge(2, 4);
    Add_edge(5, 6);
 
    // Function call
    goodsets(n);
}

Java

// Java program to find two disjoint 
// good sets of vertices in a given graph 
import java.util.*;
 
class GFG 
{
 
    static int N = 100005;
 
    // For the graph
    @SuppressWarnings("unchecked")
    static Vector<Integer>[] gr = new Vector[N], 
                            dis = new Vector[2];
    static
    {
        for (int i = 0; i < N; i++)
            gr[i] = new Vector<>();
        for (int i = 0; i < 2; i++)
            dis[i] = new Vector<>();
    }
    static boolean[] vis = new boolean[N];
    static int[] color = new int[N];
    static boolean bip;
 
    // Function to add edge
    static void add_edge(int x, int y)
    {
        gr[x].add(y);
        gr[y].add(x);
    }
 
    // Bipartite function
    static void dfs(int x, int col) 
    {
        vis[x] = true;
        color[x] = col;
 
        // Check for child vertices
        for (int i : gr[x])
        {
 
            // If it is not visited
            if (!vis[i])
                dfs(i, col ^ 1);
 
            // If it is already visited
            else if (color[i] == col)
                bip = false;
        }
    }
 
    // Function to find two disjoint
    // good sets of vertices in a given graph
    static void goodsets(int n)
    {
        // Initially assume that graph is bipartite
        bip = true;
 
        // For every unvisited vertex call dfs
        for (int i = 1; i <= n; i++)
            if (!vis[i])
                dfs(i, 0);
 
        // If graph is not bipartite
        if (!bip)
            System.out.println(-1);
        else
        {
 
            // Differentiate two sets
            for (int i = 1; i <= n; i++)
                dis[color[i]].add(i);
 
            // Print vertices belongs to both sets
 
            for (int i = 0; i < 2; i++)
            {
                for (int j = 0; j < dis[i].size(); j++)
                    System.out.print(dis[i].elementAt(j) + " ");
                System.out.println();
            }
        }
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        int n = 6, m = 4;
 
        // Add edges
        add_edge(1, 2);
        add_edge(2, 3);
        add_edge(2, 4);
        add_edge(5, 6);
 
        // Function call
        goodsets(n);
    }
}
 
// This code is contributed by
// sanjeev2552

Python3

# Python 3 program to find two disjoint
# good sets of vertices in a given graph
N = 100005
 
# For the graph
gr = [[] for i in range(N)]
dis = [[] for i in range(2)]
vis = [False for i in range(N)]
colour = [0 for i in range(N)]
bip = 0
 
# Function to add edge
def Add_edge(x, y):
    gr[x].append(y)
    gr[y].append(x)
 
# Bipartite function
def dfs(x, col):
    vis[x] = True
    colour[x] = col
 
    # Check for child vertices
    for i in gr[x]:
         
        # If it is not visited
        if (vis[i] == False):
            dfs(i, col ^ 1)
 
        # If it is already visited
        elif (colour[i] == col):
            bip = False
 
# Function to find two disjoint
# good sets of vertices in a given graph
def goodsets(n):
     
    # Initially assume that 
    # graph is bipartite
    bip = True
 
    # For every unvisited vertex call dfs
    for i in range(1, n + 1, 1):
        if (vis[i] == False):
            dfs(i, 0)
 
    # If graph is not bipartite
    if (bip == 0):
        print(-1)
    else:
         
        # Differentiate two sets
        for i in range(1, n + 1, 1):
            dis[colour[i]].append(i)
 
        # Print vertices belongs to both sets
        for i in range(2):
            for j in range(len(dis[i])):
                print(dis[i][j], end = " ") 
            print('\n', end = "")
 
# Driver code
if __name__ == '__main__':
    n = 6
    m = 4
 
    # Add edges
    Add_edge(1, 2)
    Add_edge(2, 3)
    Add_edge(2, 4)
    Add_edge(5, 6)
 
    # Function call
    goodsets(n)
 
# This code is contributed
# by Surendra_Gangwar

C#

// C# program to find two 
// disjoint good sets of 
// vertices in a given graph 
using System;
using System.Collections.Generic;
class GFG{
 
static int N = 100005;
 
// For the graph
static List<int>[] gr = 
            new List<int>[N], 
            dis = new List<int>[2];  
static bool[] vis = new bool[N];
static int[] color = new int[N];
static bool bip;
 
// Function to add edge
static void add_edge(int x, 
                     int y)
{
  gr[x].Add(y);
  gr[y].Add(x);
}
 
// Bipartite function
static void dfs(int x, 
                int col) 
{
  vis[x] = true;
  color[x] = col;
 
  // Check for child vertices
  foreach (int i in gr[x])
  {
    // If it is not visited
    if (!vis[i])
      dfs(i, col ^ 1);
 
    // If it is already visited
    else if (color[i] == col)
      bip = false;
  }
}
 
// Function to find two disjoint
// good sets of vertices in a 
// given graph
static void goodsets(int n)
{
  // Initially assume that 
  // graph is bipartite
  bip = true;
 
  // For every unvisited
  // vertex call dfs
  for (int i = 1; i <= n; i++)
    if (!vis[i])
      dfs(i, 0);
 
  // If graph is not bipartite
  if (!bip)
    Console.WriteLine(-1);
  else
  {
    // Differentiate two sets
    for (int i = 1; 
             i <= n; i++)
      dis[color[i]].Add(i);
 
    // Print vertices belongs 
    // to both sets
    for (int i = 0; i < 2; i++)
    {
      for (int j = 0; 
               j < dis[i].Count; j++)
        Console.Write(dis[i][j] + " ");
      Console.WriteLine();
    }
  }
}
 
// Driver Code
public static void Main(String[] args)
{
  int n = 6, m = 4;
   
  for (int i = 0; i < N; i++)
    gr[i] = new List<int>();
   
  for (int i = 0; i < 2; i++)
    dis[i] = new List<int>();
   
  // Add edges
  add_edge(1, 2);
  add_edge(2, 3);
  add_edge(2, 4);
  add_edge(5, 6);
 
  // Function call
  goodsets(n);
}
}
 
// This code is contributed by shikhasingrajput

Javascript

<script>
 
// JavaScript program to find two 
// disjoint good sets of 
// vertices in a given graph 
 
var N = 100005;
 
// For the graph
var gr = Array.from(Array(N), ()=>Array());
var dis = Array.from(Array(2), ()=>Array());
var vis = Array(N).fill(false);
var color = Array(N).fill(0);
var bip;
 
// Function to add edge
function add_edge(x, y)
{
  gr[x].push(y);
  gr[y].push(x);
}
 
// Bipartite function
function dfs(x, col) 
{
  vis[x] = true;
  color[x] = col;
 
  // Check for child vertices
  for(var i of gr[x])
  {
    // If it is not visited
    if (!vis[i])
      dfs(i, col ^ 1);
 
    // If it is already visited
    else if (color[i] == col)
      bip = false;
  }
}
 
// Function to find two disjoint
// good sets of vertices in a 
// given graph
function goodsets(n)
{
  // Initially assume that 
  // graph is bipartite
  bip = true;
 
  // For every unvisited
  // vertex call dfs
  for (var i = 1; i <= n; i++)
    if (!vis[i])
      dfs(i, 0);
 
  // If graph is not bipartite
  if (!bip)
    document.write(-1 + "<br>");
  else
  {
    // Differentiate two sets
    for (var i = 1; 
             i <= n; i++)
      dis[color[i]].push(i);
 
    // Print vertices belongs 
    // to both sets
    for (var i = 0; i < 2; i++)
    {
      for (var j = 0; 
               j < dis[i].length; j++)
        document.write(dis[i][j] + " ");
      document.write("<br>")
    }
  }
}
 
// Driver Code
var n = 6, m = 4;
 
// push edges
add_edge(1, 2);
add_edge(2, 3);
add_edge(2, 4);
add_edge(5, 6);
// Function call
goodsets(n);
 
</script>

Output:

1 3 4 5 
2 6

Time Complexity: O(n)

Space Complexity: O(n)

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Find two disjoint good sets of vertices in a given graph

C++

Java

Python3

C#

Javascript

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