Find a subsequence which upon reversing gives the maximum sum subarray

Given an array arr of integers of size N, the task is to find a subsequence in which upon reversing the order, the maximum sum subarray can be obtained. 

Examples:

Input: arr[] = {-2, -3, 4, -1, -2, 1, 5, -3}
Output: [-2 -3 1 5]
Explanation : After selecting subsequence -2 -3 1 5 and reverse it elements, modified array will be {5, 1, 4, -1, -2, -3, -2, -3} and thus the maximum contagious sum   i.e. 5 + 1 + 4 = 10 

Input: arr[] = {2, -6, -12, 7, -13, 9, -14}
Output: [-6 -12 7 9] 
Explanation: After selecting the above subsequence modified array will be {2, 9, 7, -12, -13, -6, -14} and thus  the maximum contagious sum i.e. is 2 + 9 + 7 = 18

Approach: The idea is simple we have to  modify the array such that all positive elements comes together, so we have to find the subsequence such that all positive elements come together when we reverse the subsequence.

• Let suppose there are " p " non- negative elements in the array. Divide the array into two parts: first p elements and the remaining elements .
• let " p^x " be  non-negative elements in first part of array. so the negative elements in the first part will be:

(size of first part of  array - number of non-negative elements) = p -  p^x

• Also number of non-negative elements in second part of array is

(total non-negative elements - non-negative elements in first part of array) =  p - p^x 

• So we have to select negative elements  p- p^x elements from first part and  p-p^x non-negative elements from the second part of array.

Below is the implementation of the above approach:

// C++ code to implement the above approach
#include <bits/stdc++.h>
using namespace std;

vector<int> findSubsequce(int arr[], int n)
{
    int p = 0;
    for (int i = 0; i < n; i++) {
        if (arr[i] >= 0)
            p++;
    }
    vector<int> res;

    // store negative elements present
    // from 0 to p-1 index
    for (int i = 0; i < p; i++) {
        if (arr[i] < 0)
            res.push_back(arr[i]);
    }

    // store non-negative elements
    // present from p to n index
    for (int i = p; i < n; i++) {
        if (arr[i] >= 0)
            res.push_back(arr[i]);
    }
    return res;
}

// Driver code
int main()
{
    int arr[] = { -2, -3, 4, -1, 
                 -2, 1, 5, -3 };
    int n = sizeof(arr) / sizeof(arr[0]);

    vector<int> res = findSubsequce(arr, n);
    for (int i = 0; i < res.size(); i++) {
        cout << res[i] << " ";
    }
}

// Java code to implement the above approach
import java.io.*;
import java.util.*;

class GFG {
    public static ArrayList<Integer>
    findSubsequence(int arr[], int n)
    {

        int p = 0;
        for (int i = 0; i < n; i++) {
            if (arr[i] >= 0)
                p++;
        }
      
        ArrayList<Integer> res
          = new ArrayList<Integer>();
      
        // store negative elements
        // present from 0 to p-1 index
        for (int i = 0; i < p; i++) {
            if (arr[i] < 0)
                res.add(arr[i]);
        }
      
        // store non-negative elements
        // present from p to n index
        for (int i = p; i < n; i++) {
            if (arr[i] >= 0)
                res.add(arr[i]);
        }
        return res;
    }

    // Driver code
    public static void main(String[] args)
    {

        int arr[] = { -2, -3, 4, -1, -2, 1, 5, -3 };
        int n = arr.length;

        ArrayList<Integer> res = findSubsequence(arr, n);

        for (int i = 0; i < res.size(); i++) {
            System.out.print(res.get(i) + " ");
        }
    }
}

# Python 3 code to implement the above approach
def findSubsequce(arr, n):

    p = 0
    for i in range(n):
        if (arr[i] >= 0):
            p += 1

    res = []

    # store negative elements present
    # from 0 to p-1 index
    for i in range(p):
        if (arr[i] < 0):
            res.append(arr[i])

    # store non-negative elements
    # present from p to n index
    for i in range(p, n):
        if (arr[i] >= 0):
            res.append(arr[i])

    return res

# Driver code
if __name__ == "__main__":

    arr = [-2, -3, 4, -1,
           -2, 1, 5, -3]
    n = len(arr)

    res = findSubsequce(arr, n)
    for i in range(len(res)):
        print(res[i], end=" ")

        # This code is contributed by ukasp.

// C# code to implement the above approach
using System;
using System.Collections;

public class GFG{

  public static ArrayList
    findSubsequence(int[] arr, int n)
  {

    int p = 0;
    for (int i = 0; i < n; i++) {
      if (arr[i] >= 0)
        p++;
    }

    var res = new ArrayList();

    // store negative elements
    // present from 0 to p-1 index
    for (int i = 0; i < p; i++) {
      if (arr[i] < 0)
        res.Add(arr[i]);
    }

    // store non-negative elements
    // present from p to n index
    for (int i = p; i < n; i++) {
      if (arr[i] >= 0)
        res.Add(arr[i]);
    }
    return res;
  }

  // Driver code
  static public void Main (){

    int[] arr = { -2, -3, 4, -1, -2, 1, 5, -3 };
    int n = arr.Length;

    ArrayList res = findSubsequence(arr, n);

    for (int i = 0; i < res.Count; i++) {
      Console.Write(res[i] + " ");
    }
  }
}

// This code is contributed by hrithikgarg03188.

 <script>
        // JavaScript code for the above approach


function findSubsequce(arr, n)
{
    let p = 0;
    for (let i = 0; i < n; i++) {
        if (arr[i] >= 0)
            p++;
    }
    let res =[];

    // store negative elements present
    // from 0 to p-1 index
    for (let i = 0; i < p; i++) {
        if (arr[i] < 0)
            res.push(arr[i]);
    }

    // store non-negative elements
    // present from p to n index
    for (let i = p; i < n; i++) {
        if (arr[i] >= 0)
            res.push(arr[i]);
    }
    return res;
}

// Driver code

    let arr = [-2, -3, 4, -1, 
                 -2, 1, 5, -3]
    let n = arr.length;

    let res = findSubsequce(arr, n);
    for (let i = 0; i < res.length; i++) {
        document.write(res[i]+ " ")
    }

       // This code is contributed by Potta Lokesh
    </script>

-2 -3 1 5 

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

Related Topic: Subarrays, Subsequences, and Subsets in Array