Largest Independent Set Problem | DP-26

Given a Binary Tree, find size of the Largest Independent Set(LIS) in it. A subset of all tree nodes is an independent set if there is no edge between any two nodes of the subset.
For example, consider the following binary tree. The largest independent set(LIS) is {10, 40, 60, 70, 80} and size of the LIS is 5.

A Dynamic Programming solution solves a given problem using solutions of subproblems in bottom up manner. Can the given problem be solved using solutions to subproblems? If yes, then what are the subproblems? Can we find largest independent set size (LISS) for a node X if we know LISS for all descendants of X? If a node is considered as part of LIS, then its children cannot be part of LIS, but its grandchildren can be. Following is optimal substructure property.



1) Optimal Substructure:
Let LISS(X) indicates size of largest independent set of a tree with root X.

     LISS(X) = MAX { (1 + sum of LISS for all grandchildren of X),
                     (sum of LISS for all children of X) }

The idea is simple, there are two possibilities for every node X, either X is a member of the set or not a member. If X is a member, then the value of LISS(X) is 1 plus LISS of all grandchildren. If X is not a member, then the value is sum of LISS of all children.

2) Overlapping Subproblems
Following is recursive implementation that simply follows the recursive structure mentioned above.

C++

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// A naive recursive implementation of
// Largest Independent Set problem 
#include <bits/stdc++.h>
using namespace std; 
  
// A utility function to find 
// max of two integers 
int max(int x, int y) 
    return (x > y) ? x : y; 
  
/* A binary tree node has data, 
pointer to left child and a 
pointer to right child */
class node 
    public:
    int data; 
    node *left, *right; 
}; 
  
// The function returns size of the 
// largest independent set in a given 
// binary tree 
int LISS(node *root) 
    if (root == NULL) 
    return 0; 
  
    // Calculate size excluding the current node 
    int size_excl = LISS(root->left) + 
                    LISS(root->right); 
  
    // Calculate size including the current node 
    int size_incl = 1; 
    if (root->left) 
        size_incl += LISS(root->left->left) +
                     LISS(root->left->right); 
    if (root->right) 
        size_incl += LISS(root->right->left) + 
                     LISS(root->right->right); 
  
    // Return the maximum of two sizes 
    return max(size_incl, size_excl); 
  
// A utility function to create a node 
node* newNode( int data ) 
    node* temp = new node();
    temp->data = data; 
    temp->left = temp->right = NULL; 
    return temp; 
  
// Driver Code
int main() 
    // Let us construct the tree 
    // given in the above diagram 
    node *root = newNode(20); 
    root->left = newNode(8); 
    root->left->left = newNode(4); 
    root->left->right = newNode(12); 
    root->left->right->left = newNode(10); 
    root->left->right->right = newNode(14); 
    root->right = newNode(22); 
    root->right->right = newNode(25); 
  
    cout << "Size of the Largest" 
         << " Independent Set is " 
         << LISS(root); 
  
    return 0; 
  
// This is code is contributed 
// by rathbhupendra

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C

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// A naive recursive implementation of Largest Independent Set problem
#include <stdio.h>
#include <stdlib.h>
  
// A utility function to find max of two integers
int max(int x, int y) { return (x > y)? x: y; }
  
/* A binary tree node has data, pointer to left child and a pointer to 
   right child */
struct node
{
    int data;
    struct node *left, *right;
};
  
// The function returns size of the largest independent set in a given 
// binary tree
int LISS(struct node *root)
{
    if (root == NULL)
       return 0;
  
    // Caculate size excluding the current node
    int size_excl = LISS(root->left) + LISS(root->right);
  
    // Calculate size including the current node
    int size_incl = 1;
    if (root->left)
       size_incl += LISS(root->left->left) + LISS(root->left->right);
    if (root->right)
       size_incl += LISS(root->right->left) + LISS(root->right->right);
  
    // Return the maximum of two sizes
    return max(size_incl, size_excl);
}
  
  
// A utility function to create a node
struct node* newNode( int data )
{
    struct node* temp = (struct node *) malloc( sizeof(struct node) );
    temp->data = data;
    temp->left = temp->right = NULL;
    return temp;
}
  
// Driver program to test above functions
int main()
{
    // Let us construct the tree given in the above diagram
    struct node *root         = newNode(20);
    root->left                = newNode(8);
    root->left->left          = newNode(4);
    root->left->right         = newNode(12);
    root->left->right->left   = newNode(10);
    root->left->right->right  = newNode(14);
    root->right               = newNode(22);
    root->right->right        = newNode(25);
  
    printf ("Size of the Largest Independent Set is %d ", LISS(root));
  
    return 0;
}

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Java

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// A naive recursive implementation of
// Largest Independent Set problem 
class GFG {
  
// A utility function to find 
// max of two integers 
static int max(int x, int y) 
    return (x > y) ? x : y; 
  
/* A binary tree node has data, 
pointer to left child and a 
pointer to right child */
static class Node 
    int data; 
    Node left, right; 
}; 
  
// The function returns size of the 
// largest independent set in a given 
// binary tree 
static int LISS(Node root) 
    if (root == null
    return 0
  
    // Calculate size excluding the current node 
    int size_excl = LISS(root.left) + 
                    LISS(root.right); 
  
    // Calculate size including the current node 
    int size_incl = 1
    if (root.left!=null
        size_incl += LISS(root.left.left) +
                    LISS(root.left.right); 
    if (root.right!=null
        size_incl += LISS(root.right.left) + 
                    LISS(root.right.right); 
  
    // Return the maximum of two sizes 
    return max(size_incl, size_excl); 
  
// A utility function to create a node 
static Node newNode( int data ) 
    Node temp = new Node();
    temp.data = data; 
    temp.left = temp.right = null
    return temp; 
  
// Driver Code
public static void main(String args[]) {
    // Let us construct the tree 
    // given in the above diagram 
    Node root = newNode(20); 
    root.left = newNode(8); 
    root.left.left = newNode(4); 
    root.left.right = newNode(12); 
    root.left.right.left = newNode(10); 
    root.left.right.right = newNode(14); 
    root.right = newNode(22); 
    root.right.right = newNode(25); 
  
    System.out.println("Size of the Largest"
        + " Independent Set is "
        + LISS(root)); 
    }
}
  
// This code has been contributed by 29AjayKumar

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C#

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// C# program for calculating LISS 
// using dynamic programming 
using System;
  
class LisTree 
    /* A binary tree node has data, pointer 
    to left child and a pointer to right 
    child */
    public class node 
    
        public int data, liss; 
        public node left, right; 
  
        public node(int data) 
        
            this.data = data; 
            this.liss = 0; 
        
    
  
    // A memoization function returns size 
    // of the largest independent set in 
    // a given binary tree 
    static int liss(node root) 
    
        if (root == null
            return 0; 
        if (root.liss != 0) 
            return root.liss; 
        if (root.left == null && root.right == null
            return root.liss = 1; 
          
        // Calculate size excluding the 
        // current node 
        int liss_excl = liss(root.left) + liss(root.right); 
          
        // Calculate size including the 
        // current node 
        int liss_incl = 1; 
        if (root.left != null
        
            liss_incl += (liss(root.left.left) + 
                        liss(root.left.right)); 
        
        if (root.right != null
        
            liss_incl += (liss(root.right.left) + 
                        liss(root.right.right)); 
        
          
        // Maximum of two sizes is LISS, 
        // store it for future uses. 
        return root.liss = Math.Max(liss_excl, liss_incl); 
    
  
    // Driver code
    public static void Main(String[] args) 
    
        // Let us construct the tree given 
        // in the above diagram 
          
        node root = new node(20); 
        root.left = new node(8); 
        root.left.left = new node(4); 
        root.left.right = new node(12); 
        root.left.right.left = new node(10); 
        root.left.right.right = new node(14); 
        root.right = new node(22); 
        root.right.right = new node(25); 
        Console.WriteLine("Size of the Largest Independent Set is " + liss(root)); 
    
  
// This code is contributed by Princi Singh

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Output:

Size of the Largest Independent Set is 5

Time complexity of the above naive recursive approach is exponential. It should be noted that the above function computes the same subproblems again and again. For example, LISS of node with value 50 is evaluated for node with values 10 and 20 as 50 is grandchild of 10 and child of 20.
Since same suproblems are called again, this problem has Overlapping Subprolems property. So LISS problem has both properties (see this and this) of a dynamic programming problem. Like other typical Dynamic Programming(DP) problems, recomputations of same subproblems can be avoided by storing the solutions to subproblems and solving problems in bottom up manner.

Following are implementation of Dynamic Programming based solution. In the following solution, an additional field ‘liss’ is added to tree nodes. The initial value of ‘liss’ is set as 0 for all nodes. The recursive function LISS() calculates ‘liss’ for a node only if it is not already set.

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/* Dynamic programming based program 
for Largest Independent Set problem */
#include <bits/stdc++.h>
using namespace std; 
  
// A utility function to find max of two integers 
int max(int x, int y) { return (x > y)? x: y; } 
  
/* A binary tree node has data, pointer
to left child and a pointer to 
right child */
class node 
    public:
    int data; 
    int liss; 
    node *left, *right; 
}; 
  
// A memoization function returns size 
// of the largest independent set in 
// a given binary tree 
int LISS(node *root) 
    if (root == NULL) 
        return 0; 
  
    if (root->liss) 
        return root->liss; 
  
    if (root->left == NULL && root->right == NULL) 
        return (root->liss = 1); 
  
    // Calculate size excluding the current node 
    int liss_excl = LISS(root->left) + LISS(root->right); 
  
    // Calculate size including the current node 
    int liss_incl = 1; 
    if (root->left) 
        liss_incl += LISS(root->left->left) + LISS(root->left->right); 
    if (root->right) 
        liss_incl += LISS(root->right->left) + LISS(root->right->right); 
  
    // Maximum of two sizes is LISS, store it for future uses. 
    root->liss = max(liss_incl, liss_excl); 
  
    return root->liss; 
  
// A utility function to create a node 
node* newNode(int data) 
    node* temp = new node(); 
    temp->data = data; 
    temp->left = temp->right = NULL; 
    temp->liss = 0; 
    return temp; 
  
// Driver code
int main() 
    // Let us construct the tree 
    // given in the above diagram 
    node *root     = newNode(20); 
    root->left         = newNode(8); 
    root->left->left     = newNode(4); 
    root->left->right     = newNode(12); 
    root->left->right->left = newNode(10); 
    root->left->right->right = newNode(14); 
    root->right         = newNode(22); 
    root->right->right     = newNode(25); 
  
    cout << "Size of the Largest Independent Set is " << LISS(root); 
  
    return 0; 
  
// This code is contributed by rathbhupendra

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/* Dynamic programming based program for Largest Independent Set problem */
#include <stdio.h>
#include <stdlib.h>
  
// A utility function to find max of two integers
int max(int x, int y) { return (x > y)? x: y; }
  
/* A binary tree node has data, pointer to left child and a pointer to 
   right child */
struct node
{
    int data;
    int liss;
    struct node *left, *right;
};
  
// A memoization function returns size of the largest independent set in
//  a given binary tree
int LISS(struct node *root)
{
    if (root == NULL)
        return 0;
  
    if (root->liss)
        return root->liss;
  
    if (root->left == NULL && root->right == NULL)
        return (root->liss = 1);
  
    // Calculate size excluding the current node
    int liss_excl = LISS(root->left) + LISS(root->right);
  
    // Calculate size including the current node
    int liss_incl = 1;
    if (root->left)
        liss_incl += LISS(root->left->left) + LISS(root->left->right);
    if (root->right)
        liss_incl += LISS(root->right->left) + LISS(root->right->right);
  
    // Maximum of two sizes is LISS, store it for future uses.
    root->liss = max(liss_incl, liss_excl);
  
    return root->liss;
}
  
// A utility function to create a node
struct node* newNode(int data)
{
    struct node* temp = (struct node *) malloc( sizeof(struct node) );
    temp->data = data;
    temp->left = temp->right = NULL;
    temp->liss = 0;
    return temp;
}
  
// Driver program to test above functions
int main()
{
    // Let us construct the tree given in the above diagram
    struct node *root         = newNode(20);
    root->left                = newNode(8);
    root->left->left          = newNode(4);
    root->left->right         = newNode(12);
    root->left->right->left   = newNode(10);
    root->left->right->right  = newNode(14);
    root->right               = newNode(22);
    root->right->right        = newNode(25);
  
    printf ("Size of the Largest Independent Set is %d ", LISS(root));
  
    return 0;
}

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// Java program for calculating LISS 
// using dynamic programming
  
public class LisTree 
{
    /* A binary tree node has data, pointer 
       to left child and a pointer to right
       child */
    static class node 
    {
        int data, liss;
        node left, right;
  
        public node(int data) 
        {
            this.data = data;
            this.liss = 0;
        }
    }
  
    // A memoization function returns size 
    // of the largest independent set in
    // a given binary tree
    static int liss(node root) 
    {
        if (root == null)
            return 0;
        if (root.liss != 0)
            return root.liss;
        if (root.left == null && root.right == null)
            return root.liss = 1;
          
        // Calculate size excluding the 
        // current node
        int liss_excl = liss(root.left) + liss(root.right);
          
        // Calculate size including the 
        // current node
        int liss_incl = 1;
        if (root.left != null
        {
            liss_incl += (liss(root.left.left) + liss(root.left.right));
        }
        if (root.right != null
        {
            liss_incl += (liss(root.right.left) + liss(root.right.right));
        }
          
        // Maximum of two sizes is LISS, 
        // store it for future uses.
        return root.liss = Math.max(liss_excl, liss_incl);
    }
  
    public static void main(String[] args) 
    {
        // Let us construct the tree given 
        // in the above diagram
          
        node root = new node(20);
        root.left = new node(8);
        root.left.left = new node(4);
        root.left.right = new node(12);
        root.left.right.left = new node(10);
        root.left.right.right = new node(14);
        root.right = new node(22);
        root.right.right = new node(25);
        System.out.println("Size of the Largest Independent Set is " + liss(root));
    }
}
  
// This code is contributed by Rishabh Mahrsee

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C#

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// C# program for calculating LISS 
// using dynamic programming
using System;
      
public class LisTree 
{
    /* A binary tree node has data, pointer 
    to left child and a pointer to right
    child */
    public class node 
    {
        public int data, liss;
        public node left, right;
  
        public node(int data) 
        {
            this.data = data;
            this.liss = 0;
        }
    }
  
    // A memoization function returns size 
    // of the largest independent set in
    // a given binary tree
    static int liss(node root) 
    {
        if (root == null)
            return 0;
        if (root.liss != 0)
            return root.liss;
        if (root.left == null && root.right == null)
            return root.liss = 1;
          
        // Calculate size excluding the 
        // current node
        int liss_excl = liss(root.left) + liss(root.right);
          
        // Calculate size including the 
        // current node
        int liss_incl = 1;
        if (root.left != null
        {
            liss_incl += (liss(root.left.left) + liss(root.left.right));
        }
        if (root.right != null
        {
            liss_incl += (liss(root.right.left) + liss(root.right.right));
        }
          
        // Maximum of two sizes is LISS, 
        // store it for future uses.
        return root.liss = Math.Max(liss_excl, liss_incl);
    }
  
    // Driver code
    public static void Main(String[] args) 
    {
        // Let us construct the tree given 
        // in the above diagram
          
        node root = new node(20);
        root.left = new node(8);
        root.left.left = new node(4);
        root.left.right = new node(12);
        root.left.right.left = new node(10);
        root.left.right.right = new node(14);
        root.right = new node(22);
        root.right.right = new node(25);
        Console.WriteLine("Size of the Largest Independent Set is " + liss(root));
    }
}
  
/* This code is contributed by PrinciRaj1992 */
// C# program for calculating LISS 
// using dynamic programming
using System;
      
public class LisTree 
{
    /* A binary tree node has data, pointer 
    to left child and a pointer to right
    child */
    public class node 
    {
        public int data, liss;
        public node left, right;
  
        public node(int data) 
        {
            this.data = data;
            this.liss = 0;
        }
    }
  
    // A memoization function returns size 
    // of the largest independent set in
    // a given binary tree
    static int liss(node root) 
    {
        if (root == null)
            return 0;
        if (root.liss != 0)
            return root.liss;
        if (root.left == null && root.right == null)
            return root.liss = 1;
          
        // Calculate size excluding the 
        // current node
        int liss_excl = liss(root.left) + liss(root.right);
          
        // Calculate size including the 
        // current node
        int liss_incl = 1;
        if (root.left != null
        {
            liss_incl += (liss(root.left.left) + liss(root.left.right));
        }
        if (root.right != null
        {
            liss_incl += (liss(root.right.left) + liss(root.right.right));
        }
          
        // Maximum of two sizes is LISS, 
        // store it for future uses.
        return root.liss = Math.Max(liss_excl, liss_incl);
    }
  
    // Driver code
    public static void Main(String[] args) 
    {
        // Let us construct the tree given 
        // in the above diagram
          
        node root = new node(20);
        root.left = new node(8);
        root.left.left = new node(4);
        root.left.right = new node(12);
        root.left.right.left = new node(10);
        root.left.right.right = new node(14);
        root.right = new node(22);
        root.right.right = new node(25);
        Console.WriteLine("Size of the Largest Independent Set is " + liss(root));
    }
}
  
/* This code is contributed by PrinciRaj1992 */

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Output:

Size of the Largest Independent Set is 5


Time Complexity:
O(n) where n is the number of nodes in given Binary tree.

Following extensions to above solution can be tried as an exercise.
1) Extend the above solution for n-ary tree.

2) The above solution modifies the given tree structure by adding an additional field ‘liss’ to tree nodes. Extend the solution so that it doesn’t modify the tree structure.

3) The above solution only returns size of LIS, it doesn’t print elements of LIS. Extend the solution to print all nodes that are part of LIS.

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