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Cost of creating smallest subsequence with sum of difference between adjacent elements maximum
  • Last Updated : 14 May, 2021

Given two array of N integers arr[] and costArray[], representing removal cost associated with each element. The task is to find the subsequence from the given array of minimum cost such that the sum of the difference between adjacent elements is maximum. On removing each element, cost is incurred.
Examples:

Input: N = 3, arr[] = { 3, 2, 1 }, costArray[] = {0, 1, 0} 
Output: 4, 1 
Explanation: 
There are 4 subsequences of length at least 2 : 
[ 3, 2 ] which gives us | 3 – 2 | = 1 
[ 3, 1 ] which gives us | 3 – 1 | = 2 
[ 2, 1 ] which gives us | 2 – 1 | = 1 
[ 3, 2, 1 ] which gives us | 3 – 2 | + | 2 – 1 | = 2 
So the answer is either [ 3, 1 ] or [ 3, 2, 1 ] . Since we want the subsequence to be as short as possible, the answer is [ 3, 1 ]. Cost incurrent removing element 2 is 1. 
So, the sum of the sequence is 4. 
and the cost is 1.
Input: N = 4, arr[] = { 1, 3, 4, 2}, costArray[] = {0, 1, 0, 0} 
Output: 7, 1

Naive Approach: 
The naive approach is simply to check all the subsequences by generating them by recursion and it takes the complexity of O(2^N) which is very high complexity. And from them choose the subsequence which follows the above condition with maximum absolute sum and a minimum length as mentioned above.
Efficient Approach:

  1. We can observe the pattern, let us assume three numbers a, b, c such that a = L, b = L + 6, c = L + 10 (L is any integer here). If they are in a continuous pattern one after another eg a, b, c (such that a < b < c).
  2. Then

| b – a | + | c – b | = | a – c | = 10

  1. here we can remove the middle element to reduce the size of the original sequence.
  2. In this way, reducing the size of the array by removing a middle element from the sequence will not affect the sum and also reduce the length of the sequence.
  3. Stores all the removed elements in the set. Add the cost of removed elements. Finally, calculate the sum sequence by excluding removed elements.
  4. Then print the sum of elements of subsequence and cost incurred.

In this way, we reduce the exponential complexity O(2^N) to the linear complexity O(N).



Below is the implementation of the above approach:

C++




#include <bits/stdc++.h>
using namespace std;
 
void costOfSubsequence(
    int N, int arr[],
    int costArray[])
{
    int i, temp;
    // initializing cost=0
    int cost = 0;
 
    // to store the removed
    // element
    set<int> removedElements;
 
    // this will store the sum
    // of the subsequence
    int ans = 0;
 
    // checking all the element
    // of the vector
    for (i = 1; i < (N - 1); i++) {
        // storing the value of
        // arr[i] in temp variable
        temp = arr[i];
 
        // if the situation like
        // arr[i-1]<arr[i]<arr[i+1] or
        // arr[i-1]>arr[i]>arr[i+1] occur
        // remove arr[i] i.e, temp
        // from sequence
        if (((arr[i - 1] < temp)
             && (temp < arr[i + 1]))
            || ((arr[i - 1] > temp)
                && (temp > arr[i + 1]))) {
            // insert the element in the set
            // removedElements
            removedElements.insert(temp);
        }
    }
    for (i = 0; i < (N); i++) {
        // storing the value of
        // arr[i] in temp
        temp = arr[i];
        // taking the element not
        // in removedElements
        if (!(removedElements.count(temp) > 0)) {
            // adding the value of elements
            // of subsequence
            ans += arr[i];
        }
        else {
            // if we have to remove
            // the element then we
            // need to add the  cost
            // associated with the
            // element
            cost += costArray[i];
        }
    }
 
    // printing the sum of
    // the subsecquence with
    // minimum length possible
    cout << ans << ", ";
    // printing the cost incurred
    // in creating subsequence
    cout << cost << endl;
}
 
// Driver code
int main()
{
 
    int N;
    N = 4;
    int arr[N]
        = { 1, 3, 4, 2 };
    int costArray[N]
        = { 0, 1, 0, 0 };
 
    // calling the function
    costOfSubsequence(
        N, arr,
        costArray);
 
    return 0;
}

Java




import java.util.*;
class GFG{
     
public static void costOfSubsequence(int N, int[] arr,
                                     int[] costArray)
{
    int i, temp;
     
    // Initializing cost=0
    int cost = 0;
     
    // To store the removed
    // element
    Set<Integer> removedElements = new HashSet<Integer>();
     
    // This will store the sum
    // of the subsequence
    int ans = 0;
     
    // Checking all the element
    // of the vector
    for(i = 1; i < (N - 1); i++)
    {
         
       // Storing the value of
       // arr[i] in temp variable
       temp = arr[i];
        
       // If the situation like
       // arr[i-1]<arr[i]<arr[i+1] or
       // arr[i-1]>arr[i]>arr[i+1] occur
       // remove arr[i] i.e, temp
       // from sequence
       if (((arr[i - 1] < temp) &&
          (temp < arr[i + 1])) ||
           ((arr[i - 1] > temp) &&
          (temp > arr[i + 1])))
       {
            
           // Insert the element in the set
           // removedElements
           removedElements.add(temp);
       }
    }
    for(i = 0; i < (N); i++)
    {
        
       // Storing the value of
       // arr[i] in temp
       temp = arr[i];
        
       // Taking the element not
       // in removedElements
       if (!(removedElements.contains(temp)))
       {
            
           // Adding the value of elements
           // of subsequence
           ans += arr[i];
       }
       else
       {
            
           // If we have to remove
           // the element then we
           // need to add the cost
           // associated with the
           // element
           cost += costArray[i];
       }
    }
     
    // Printing the sum of
    // the subsecquence with
    // minimum length possible
    System.out.print(ans + ", ");
     
    // Printing the cost incurred
    // in creating subsequence
    System.out.print(cost);
}
 
// Driver code
public static void main(String[] args)
{
    int N;
    N = 4;
     
    int[] arr = { 1, 3, 4, 2 };
    int[] costArray = { 0, 1, 0, 0 };
     
    // Calling the function
    costOfSubsequence(N, arr, costArray);
}
}
 
// This code is contributed by divyeshrabadiya07

Python3




def costOfSubsequence(N, arr, costArray):
     
    # Initializing cost=0
    i, temp, cost = 0, 0, 0
 
    # To store the removed
    # element
    removedElements = {''}
 
    # This will store the sum
    # of the subsequence
    ans = 0
 
    # Checking all the element
    # of the vector
    for i in range(1, N - 1):
         
        # Storing the value of
        # arr[i] in temp variable
        temp = arr[i]
 
        # If the situation like
        # arr[i-1]<arr[i]<arr[i+1] or
        # arr[i-1]>arr[i]>arr[i+1] occur
        # remove arr[i] i.e, temp
        # from sequence
        if (((arr[i - 1] < temp) and
            (temp < arr[i + 1])) or
            ((arr[i - 1] > temp) and
            (temp > arr[i + 1]))) :
                 
            # Insert the element in the set
            # removedElements
            removedElements.add(temp)
 
    for i in range(0, N):
         
        # Storing the value of
        # arr[i] in temp
        temp = arr[i]
         
        # Taking the element not
        # in removedElements
        if(temp not in removedElements):
             
            # Adding the value of elements
            # of subsequence
            ans = ans + arr[i]
        else:
             
            # If we have to remove
            # the element then we
            # need to add the cost
            # associated with the
            # element
            cost += costArray[i]
 
    # Printing the sum of
    # the subsequence with
    # minimum length possible
    print(ans, end = ", ")
     
    # Printing the cost incurred
    # in creating subsequence
    print(cost)
 
# Driver code
N = 4
arr = [ 1, 3, 4, 2 ]
costArray = [ 0, 1, 0, 0 ]
 
# Calling the function
costOfSubsequence(N, arr, costArray)
 
# This code is contributed by Sanjit_Prasad

C#




using System;
using System.Collections.Generic;
 
class GFG{
     
public static void costOfSubsequence(int N, int[] arr,
                                     int[] costArray)
{
    int i, temp;
     
    // Initializing cost=0
    int cost = 0;
     
    // To store the removed
    // element
    HashSet<int> removedElements = new HashSet<int>();
     
    // This will store the sum
    // of the subsequence
    int ans = 0;
     
    // Checking all the element
    // of the vector
    for(i = 1; i < (N - 1); i++)
    {
         
        // Storing the value of
        // arr[i] in temp variable
        temp = arr[i];
             
        // If the situation like
        // arr[i-1]<arr[i]<arr[i+1] or
        // arr[i-1]>arr[i]>arr[i+1] occur
        // remove arr[i] i.e, temp
        // from sequence
        if (((arr[i - 1] < temp) &&
            (temp < arr[i + 1])) ||
            ((arr[i - 1] > temp) &&
            (temp > arr[i + 1])))
        {
                 
            // Insert the element in the set
            // removedElements
            removedElements.Add(temp);
        }
    }
    for(i = 0; i < (N); i++)
    {
         
        // Storing the value of
        // arr[i] in temp
        temp = arr[i];
             
        // Taking the element not
        // in removedElements
        if (!(removedElements.Contains(temp)))
        {
             
            // Adding the value of elements
            // of subsequence
            ans += arr[i];
        }
        else
        {
             
            // If we have to remove
            // the element then we
            // need to add the cost
            // associated with the
            // element
            cost += costArray[i];
        }
    }
     
    // Printing the sum of
    // the subsequence with
    // minimum length possible
    Console.Write(ans + ", ");
     
    // Printing the cost incurred
    // in creating subsequence
    Console.Write(cost);
}
 
// Driver code
public static void Main(String[] args)
{
    int N;
    N = 4;
     
    int[] arr = { 1, 3, 4, 2 };
    int[] costArray = { 0, 1, 0, 0 };
     
    // Calling the function
    costOfSubsequence(N, arr, costArray);
}
}
 
// This code is contributed by Amit Katiyar
Output: 
7, 1

Time Complexity: O(N)
 

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