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Maximal Independent Set in an Undirected Graph

  • Last Updated : 13 Jul, 2020

Given an undirected graph defined by the number of vertex V and the edges E[ ], the task is to find Maximal Independent Vertex Set in an undirected graph.

Independent Set: An independent set in a graph is a set of vertices which are not directly connected to each other.

Note: It is a given that there is at least one way to traverse from any vertex in the graph to another, i.e. the graph has one connected component.


Input: V = 3, E = { (1, 2), (2, 3) }
Output: {1, 3}
Since there are no edges between 1 and 3, and we cannot add 2 to this since it is a neighbour of 1, this is the Maximal Independent Set.

Input: V = 8,
E = { (1, 2), (1, 3), (2, 4), (5, 6), (6, 7), (4, 8) }
Output: {2, 3, 5, 7, 8}

This problem is an NP-Hard problem, which can only be solved in exponential time(as of right now).
Follow the steps below to solve the problem:

  • Iterate through the vertices of the graph and use backtracking to check if a vertex can be included in the Maximal Independent Set or not.
  • Two possibilities arise for each vertex, whether it can be included or not in the maximal independent set.
  • Initially start, considering all vertices and edges. One by one, select a vertex. Remove that vertex from the graph, excluding it from the maximal independent set, and recursively traverse the remaining graph to find the maximal independent set.
  • Otherwise, consider the selected vertex in the maximal independent set and remove all its neighbors from it. Proceed to find the maximal independent set possible excluding its neighbors.
  • Repeat this process for all vertices and print the maximal independent set obtained.

Below is the implementation of the above approach:


# Python Program to implement
# the above approach
# Recursive Function to find the
# Maximal Independent Vertex Set    
def graphSets(graph):
    # Base Case - Given Graph 
    # has no nodes
    if(len(graph) == 0):
        return []
    # Base Case - Given Graph
    # has 1 node
    if(len(graph) == 1):
        return [list(graph.keys())[0]]
    # Select a vertex from the graph
    vCurrent = list(graph.keys())[0]
    # Case 1 - Proceed removing
    # the selected vertex
    # from the Maximal Set
    graph2 = dict(graph)
    # Delete current vertex 
    # from the Graph
    del graph2[vCurrent]
    # Recursive call - Gets 
    # Maximal Set,
    # assuming current Vertex 
    # not selected
    res1 = graphSets(graph2)
    # Case 2 - Proceed considering
    # the selected vertex as part
    # of the Maximal Set
    # Loop through its neighbours
    for v in graph[vCurrent]:
        # Delete neighbor from 
        # the current subgraph
        if(v in graph2):
            del graph2[v]
    # This result set contains VFirst,
    # and the result of recursive
    # call assuming neighbors of vFirst
    # are not selected
    res2 = [vCurrent] + graphSets(graph2)
    # Our final result is the one 
    # which is bigger, return it
    if(len(res1) > len(res2)):
        return res1
    return res2
# Driver Code
V = 8
# Defines edges
E = [ (1, 2),
      (1, 3),
      (2, 4),
      (5, 6),
      (6, 7),
      (4, 8)]
graph = dict([])
# Constructs Graph as a dictionary 
# of the following format-
# graph[VertexNumber V] 
# = list[Neighbors of Vertex V]
for i in range(len(E)):
    v1, v2 = E[i]
    if(v1 not in graph):
        graph[v1] = []
    if(v2 not in graph):
        graph[v2] = []
# Recursive call considering 
# all vertices in the maximum 
# independent set
maximalIndependentSet = graphSets(graph)
# Prints the Result 
for i in maximalIndependentSet:
    print(i, end =" ")
2 3 8 5 7

Time Complexity: O(2N)
Auxiliary Space: O(N)

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