Longest alternating subsequence with maximum sum | Set 2

Given an array arr[] of size N, consisting of positive and negative integers, the task is to find the longest alternating subsequence(i.e. the sign of every element is opposite to that of its previous element) from the given array which has the maximum sum.
Examples: 
 

Input: arr[] = {-2, 10, 3, -8, -4, -1, 5, -2, -3, 1} 
Output: 11 
Explanation: 
Since the subsequence needs to be longest possible as well as alternating, one element can be selected from each of the following subarrays: 
{-2}, {10, 3}, {-8, -4, -1}, {5}, {-2, -3}, {1} 
Hence, selecting the maximum from each of the subarrays as the elements of the subsequence generates an alternating subsequence with maximum sum. 
Therefore, the subsequence is {-2, 10, -1, 5, -2, 1} 
Hence, the sum of the subsequence is 11.
Input: arr[] = {12, 4, -5, 7, -9} 
Output:
Explanation: 
The longest subsequence with greatest sum is {12, -5, 7, -9}. 
Hence, the maximum sum is 5. 
 

 

Linear Approach using extra-space: 
Refer to Longest alternating subsequence which has maximum sum of elements for the linear approach using extra space. 
Time Complexity: O(N) 
Auxiliary Space: O(N)
Space-Efficient Approach: 
To solve the problem, we can observe the following: 
 

  • To maximize the length of the alternating subsequence, we need to consider an element from every sequence of consecutive numbers of the 
     

Illustration: 
Let us consider an array arr[] = {1, 1, 2, -1, -5, 2, 1, -3} 
The consecutive sequences of elements of same sign are: 
{1, 1, 2}, {-1, -5}, {2, 1}, {-3} 
Hence, by selecting an element from each of these sequences, an alternating subsequence of the longest possible length can be obtained. 
 



  • To maximize the sum of the subsequence, we need to select the maximum from each consecutive subsequence of elements of the same sign. 
     

Illustration: 
For the array arr[] = {1, 1, 2, -1, -5, 2, 1, -3}, the consecutive sequences of elements of sign were observed to be: 
{1, 1, 2}, {-1, -5}, {2, 1}, {-3} 
Therefore, the subsequence with the maximum sum is {2, -1, 2, -3}, formed by selecting the maximum element from each of the sequences. 
 

 

Follow the steps below to solve the problem efficiently: 
 

  • Iterate over the array using Two Pointers.
  • Set i = 0, and set j = i.
  • Traverse the array until j points to an index consisting of an element of sign opposite to that of arr[i]. At every traversal, update the maximum element encountered between [i, j].
  • Once an element of opposite sign is found, add the maximum from the sequence [i, j) to maxsum.
  • Set i = j, and repeat the above two steps until the entire array is traversed.
  • Print the final value of maxsum as the answer.

Below is the implementation of the above approach:
 

C++

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// C++ Program to implement
// the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to check the
// sign of the element
int sign(int x)
{
    if (x > 0)
        return 1;
    else
        return -1;
}
 
// Function to calculate and
// return the maximum sum of
// longest alternating subsequence
int findMaxSum(int arr[], int size)
{
    int max_sum = 0, pres, i, j;
 
    // Iterate through the array
    for (i = 0; i < size; i++) {
 
        // Stores the first element of
        // a sequence of same sign
        pres = arr[i];
        j = i;
 
        // Traverse until an element with
        // opposite sign is encountered
        while (j < size
               && sign(arr[i])
                      == sign(arr[j])) {
 
            // Update the maximum
            pres = max(pres, arr[j]);
            j++;
        }
 
        // Update the maximum sum
        max_sum = max_sum + pres;
 
        // Update i
        i = j - 1;
    }
 
    // Return the maximum sum
    return max_sum;
}
 
// Driver Code
int main()
{
    int arr[] = { -2, 8, 3, 8, -4,
                  -15, 5, -2, -3, 1 };
 
    int size = sizeof(arr)
/ sizeof(arr[0]);
 
    cout << findMaxSum(arr, size);
 
    return 0;
}

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Java

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// Java Program to implement
// the above approach
import java.util.*;
class GFG{
 
    // Function to check the
    // sign of the element
    static int sign(int x)
    {
        if (x > 0)
            return 1;
        else
            return -1;
    }
 
    // Function to calculate and
    // return the maximum sum of
    // longest alternating subsequence
    static int findMaxSum(int arr[], int size)
    {
        int max_sum = 0, pres, i, j;
 
        // Iterate through the array
        for (i = 0; i < size; i++)
        {
 
            // Stores the first element of
            // a sequence of same sign
            pres = arr[i];
            j = i;
 
            // Traverse until an element with
            // opposite sign is encountered
            while (j < size &&
                   sign(arr[i]) == sign(arr[j]))
            {
 
                // Update the maximum
                pres = Math.max(pres, arr[j]);
                j++;
            }
 
            // Update the maximum sum
            max_sum = max_sum + pres;
 
            // Update i
            i = j - 1;
        }
 
        // Return the maximum sum
        return max_sum;
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        int arr[] = { -2, 8, 3, 8, -4, -15, 5, -2, -3, 1 };
 
        int size = arr.length;
 
        System.out.println(findMaxSum(arr, size));
    }
}
 
// This code is contributed by sapnasingh4991

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Python

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# Python3 program to implement
# the above approach
 
# Function to check the
# sign of the element
def sign(x):
 
    if (x > 0):
        return 1
    else:
        return -1
 
# Function to calculate and
# return the maximum sum of
# longest alternating subsequence
def findMaxSum(arr, size):
     
    max_sum = 0
 
    # Iterate through the array
    i = 0
    while i < size:
 
        # Stores the first element of
        # a sequence of same sign
        pres = arr[i]
        j = i
 
        # Traverse until an element with
        # opposite sign is encountered
        while (j < size and
              (sign(arr[i]) == sign(arr[j]))):
 
            # Update the maximum
            pres = max(pres, arr[j])
            j += 1
 
        # Update the maximum sum
        max_sum = max_sum + pres
 
        # Update i
        i = j - 1
        i += 1
 
    # Return the maximum sum
    return max_sum
 
# Driver Code
if __name__ == "__main__":
     
    arr = [ -2, 8, 3, 8, -4,
            -15, 5, -2, -3, 1 ]
 
    size = len(arr)
 
    print(findMaxSum(arr, size))
 
# This code is contributed by chitranayal

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C#

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// C# Program to implement
// the above approach
using System;
class GFG{
 
    // Function to check the
    // sign of the element
    static int sign(int x)
    {
        if (x > 0)
            return 1;
        else
            return -1;
    }
 
    // Function to calculate and
    // return the maximum sum of
    // longest alternating subsequence
    static int findMaxSum(int []arr, int size)
    {
        int max_sum = 0, pres, i, j;
 
        // Iterate through the array
        for (i = 0; i < size; i++)
        {
 
            // Stores the first element of
            // a sequence of same sign
            pres = arr[i];
            j = i;
 
            // Traverse until an element with
            // opposite sign is encountered
            while (j < size &&
                   sign(arr[i]) == sign(arr[j]))
            {
 
                // Update the maximum
                pres = Math.Max(pres, arr[j]);
                j++;
            }
 
            // Update the maximum sum
            max_sum = max_sum + pres;
 
            // Update i
            i = j - 1;
        }
 
        // Return the maximum sum
        return max_sum;
    }
 
    // Driver Code
    public static void Main(String[] args)
    {
        int []arr = { -2, 8, 3, 8, -4,
                     -15, 5, -2, -3, 1 };
 
        int size = arr.Length;
 
        Console.WriteLine(findMaxSum(arr, size));
    }
}
 
// This code is contributed by gauravrajput1

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Output:

6






Time Complexity: O(N) 
Auxiliary Space: O(1)

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