Maximum subarray sum possible after removing at most K array elements

Given an array arr[] of size N and an integer K, the task is to find the maximum subarray sum by removing at most K elements from the array.

Examples:

Input: arr[] = { -2, 1, 3, -2, 4, -7, 20 }, K = 1 
Output: 26 
Explanation:
Removing arr[5] from the array modifies arr[] to { -2, 1, 3, -2, 4, 20 } 
Subarray with maximum sum is { 1, 3, -2, 4, 20 }. 
Therefore, the required output is 26.

Input:arr[] = { -1, 1, -1, -1, 1, 1 }, K=2 
Output: 3 
Explanation:
Removing arr[2] and arr[3] from the array modifies arr[] to { – 1, 1, 1, 1} 
Subarray with maximum sum is { 1, 1, 1 }. 
Therefore, the required output is 3.

Approach: The problem can be solved using Dynamic Programming. The idea is to use the concept of Kadane’s algorithm. Follow the steps below to solve the problem: 

• Traverse the array arr[] and for every array element following two operations needs to be performed: 
  • Remove the current array element from the subarray.
  • Include the current array element in the subarray.
• Therefore, the recurrence relation to solve this problem is as follows:

mxSubSum(i, j) = max(max(0, arr[i] + mxSubSum(i – 1, j)), mxSubSum(i – 1, j – 1))

i: Stores index of array element 
j: Maximum count of elements that can be removed from the subarray 
mxSubSum(i, j): Return maximum subarray sum from the subarray { arr[i], arr[N – 1] } by removing K – j array elements.

• Initialize a 2D array, say dp[][], to store the overlapping subproblems of the above recurrence relation.
• Fill the dp[][] array using memoization.
• Find the maximum element from the dp[][] array say, res.
• Initialize a variable, say Max, to store the largest element present in the array arr[].
• If all the array elements are negative, then update res = max(res, Max).
• Finally, print res as the required answer.

Below is the implementation of the above approach:

C++

// C++ program to implement
// the above approach

#include <bits/stdc++.h>
using namespace std;
#define M 100

// Function to find the maximum subarray
// sum greater than or equal to 0 by
// removing K array elements
int mxSubSum(int i, int* arr,
             int j, int dp[][M])
{
    // Base case
    if (i == 0) {
        return dp[i][j] = max(0, arr[i]);
    }

    // If overlapping subproblems
    // already occurred
    if (dp[i][j] != -1) {
        return dp[i][j];
    }

    // Include current element in the subarray
    int X = max(0, arr[i]
                       + mxSubSum(i - 1, arr, j, dp));

    // If K elements already removed
    // from the subarray
    if (j == 0) {

        return dp[i][j] = X;
    }

    // Remove current element from the subarray
    int Y = mxSubSum(i - 1, arr, j - 1, dp);

    return dp[i][j] = max(X, Y);
}

// Utility function to find the maximum subarray
// sum by removing at most K array elements
int MaximumSubarraySum(int n, int* arr, int k)
{

    // Stores overlapping subproblems
    // of the recurrence relation
    int dp[M][M];

    // Initialize dp[][] to -1
    memset(dp, -1, sizeof(dp));

    mxSubSum(n - 1, arr, k, dp);

    // Stores maximum subarray sum by
    // removing at most K elements
    int res = 0;

    // Calculate maximum element
    // in dp[][]
    for (int i = 0; i < n; i++) {
        for (int j = 0; j <= k; j++) {

            // Update res
            res = max(res, dp[i][j]);
        }
    }

    // If all array elements are negative
    if (*max_element(arr, arr + n) < 0) {

        // Update res
        res = *max_element(arr, arr + n);
    }
    return res;
}

// Driver Code
int main()
{
    int arr[] = { -2, 1, 3, -2, 4, -7, 20 };
    int K = 1;
    int N = sizeof(arr) / sizeof(arr[0]);
    cout << MaximumSubarraySum(N, arr, K) << endl;

    return 0;
}

Java

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

class GFG{
static final int M = 100;

// Function to find the maximum subarray
// sum greater than or equal to 0 by
// removing K array elements
static int mxSubSum(int i, int []arr,
             int j, int dp[][])
{
    // Base case
    if (i == 0) {
        return dp[i][j] = Math.max(0, arr[i]);
    }

    // If overlapping subproblems
    // already occurred
    if (dp[i][j] != -1) {
        return dp[i][j];
    }

    // Include current element in the subarray
    int X = Math.max(0, arr[i]
                       + mxSubSum(i - 1, arr, j, dp));

    // If K elements already removed
    // from the subarray
    if (j == 0)
    {
        return dp[i][j] = X;
    }

    // Remove current element from the subarray
    int Y = mxSubSum(i - 1, arr, j - 1, dp);

    return dp[i][j] = Math.max(X, Y);
}

// Utility function to find the maximum subarray
// sum by removing at most K array elements
static int MaximumSubarraySum(int n, int []arr, int k)
{

    // Stores overlapping subproblems
    // of the recurrence relation
    int [][]dp = new int[M][M];

    // Initialize dp[][] to -1
    for (int i = 0; i < M; i++)
        for (int j = 0; j < M; j++)
            dp[i][j] = -1;

    mxSubSum(n - 1, arr, k, dp);

    // Stores maximum subarray sum by
    // removing at most K elements
    int res = 0;

    // Calculate maximum element
    // in dp[][]
    for (int i = 0; i < n; i++) {
        for (int j = 0; j <= k; j++) {

            // Update res
            res = Math.max(res, dp[i][j]);
        }
    }

    // If all array elements are negative
    if (Arrays.stream(arr).max().getAsInt() < 0) {

        // Update res
        res = Arrays.stream(arr).max().getAsInt();
    }
    return res;
}

// Driver Code
public static void main(String[] args)
{
    int arr[] = { -2, 1, 3, -2, 4, -7, 20 };
    int K = 1;
    int N = arr.length;
    System.out.print(MaximumSubarraySum(N, arr, K) +"\n");
}
}

// This code is contributed by 29AjayKumar

Python3

# Python3 program to implement
# the above approach
M = 100

# Function to find the maximum subarray
# sum greater than or equal to 0 by
# removing K array elements
def mxSubSum(i, arr, j):
    global dp

    # Base case
    if (i == 0):
        dp[i][j] = max(0, arr[i])
        return dp[i][j]

    # If overlapping subproblems
    # already occurred
    if (dp[i][j] != -1):
        return dp[i][j]

    # Include current element in the subarray
    X = max(0, arr[i] + mxSubSum(i - 1, arr, j))

    # If K elements already removed
    # from the subarray
    if (j == 0):

        dp[i][j] = X
        return X

    # Remove current element from the subarray
    Y = mxSubSum(i - 1, arr, j - 1)

    dp[i][j] = max(X, Y)

    return dp[i][j]

# Utility function to find the maximum subarray
# sum by removing at most K array elements
# Utility function to find the maximum subarray
# sum by removing at most K array elements
def MaximumSubarraySum(n, arr, k):

    mxSubSum(n - 1, arr, k)

    # Stores maximum subarray sum by
    # removing at most K elements
    res = 0

    # Calculate maximum element
    # in dp[][]
    for i in range(n):
        for j in range(k + 1):

            # Update res
            res = max(res, dp[i][j])

    # If all array elements are negative
    if (max(arr) < 0):

        # Update res
        res = max(arr)
    return res

# Driver Code
if __name__ == '__main__':
    dp = [[-1 for i in range(100)] for i in range(100)]
    arr = [-2, 1, 3, -2, 4, -7, 20]
    K = 1
    N = len(arr)
    print(MaximumSubarraySum(N, arr, K))

# This code is contributed by mohit kumar 29

C#

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

class GFG{

static int M = 100;

// Function to find the maximum subarray
// sum greater than or equal to 0 by
// removing K array elements
static int mxSubSum(int i, int []arr,
                    int j, int [,]dp)
{

    // Base case
    if (i == 0)
    {
        return dp[i, j] = Math.Max(0, arr[i]);
    }

    // If overlapping subproblems
    // already occurred
    if (dp[i, j] != -1)
    {
        return dp[i, j];
    }

    // Include current element in the subarray
    int X = Math.Max(0, arr[i] +
                    mxSubSum(i - 1, arr, j, dp));

    // If K elements already removed
    // from the subarray
    if (j == 0)
    {
        return dp[i, j] = X;
    }

    // Remove current element from the subarray
    int Y = mxSubSum(i - 1, arr, j - 1, dp);

    return dp[i, j] = Math.Max(X, Y);
}

// Utility function to find the maximum subarray
// sum by removing at most K array elements
static int MaximumSubarraySum(int n, int []arr, int k)
{

    // Stores overlapping subproblems
    // of the recurrence relation
    int [,]dp = new int[M, M];

    // Initialize dp[,] to -1
    for(int i = 0; i < M; i++)
        for(int j = 0; j < M; j++)
            dp[i, j] = -1;

    mxSubSum(n - 1, arr, k, dp);

    // Stores maximum subarray sum by
    // removing at most K elements
    int res = 0;

    // Calculate maximum element
    // in dp[,]
    for(int i = 0; i < n; i++)
    {
        for(int j = 0; j <= k; j++)
        {

            // Update res
            res = Math.Max(res, dp[i, j]);
        }
    }

    Array.Sort(arr);

    // If all array elements are negative

    if (arr[n - 1] < 0)
    {

        // Update res
        res = arr[n - 1];
    }
    return res;
}

// Driver Code
public static void Main(String[] args)
{
    int []arr = { -2, 1, 3, -2, 4, -7, 20 };
    int K = 1;
    int N = arr.Length;

    Console.WriteLine(MaximumSubarraySum(N, arr, K));
}
}

// This code is contributed by AnkThon

Javascript

<script>

// Javascript program to implement
// the above approach
var M = 100;

// Function to find the maximum subarray
// sum greater than or equal to 0 by
// removing K array elements
function mxSubSum(i, arr, j, dp)
{

    // Base case
    if (i == 0)
    {
        dp[i][j] = Math.max(0, arr[i]);
        return dp[i][j];
    }

    // If overlapping subproblems
    // already occurred
    if (dp[i][j] != -1)
    {
        return dp[i][j];
    }

    // Include current element in the subarray
    var X = Math.max(
        0, arr[i] + mxSubSum(i - 1, arr, j, dp));

    // If K elements already removed
    // from the subarray
    if (j == 0)
    {
        dp[i][j] = X;
        return dp[i][j]
    }

    // Remove current element from the subarray
    var Y = mxSubSum(i - 1, arr, j - 1, dp);

    dp[i][j] = Math.max(X, Y);
    return dp[i][j]
}

// Utility function to find the maximum subarray
// sum by removing at most K array elements
function MaximumSubarraySum(n, arr, k)
{

    // Stores overlapping subproblems
    // of the recurrence relation
    var dp = Array.from(Array(M), () => Array(M).fill(-1));

    mxSubSum(n - 1, arr, k, dp);

    // Stores maximum subarray sum by
    // removing at most K elements
    var res = 0;

    // Calculate maximum element
    // in dp[][]
    for(var i = 0; i < n; i++)
    {
        for(var j = 0; j <= k; j++)
        {

            // Update res
            res = Math.max(res, dp[i][j]);
        }
    }

    // If all array elements are negative
    if (arr.reduce((a, b) => Math.max(a, b)) < 0)
    {

        // Update res
        res = arr.reduce((a, b) => Math.max(a, b));
    }
    return res;
}

// Driver Code
var arr = [ -2, 1, 3, -2, 4, -7, 20 ];
var K = 1;
var N = arr.length;

document.write( MaximumSubarraySum(N, arr, K));

// This code is contributed by rrrtnx

</script>

Output:

26

Time Complexity: O(N * K)
Auxiliary Space: O(N * K)

Efficient approach : Using DP Tabulation method ( Iterative approach )

The approach to solve this problem is same but DP tabulation(bottom-up) method is better then Dp + memorization(top-down) because memorization method needs extra stack space of recursion calls.

Steps to solve this problem :

• Create a vector to store the solution of the subproblems.
• Initialize the table with base cases
• Fill up the table iteratively
• Return the final solution

Implementation :

C++

// C++ program to implement
// the above approach

#include <bits/stdc++.h>
using namespace std;
#define M 100

// Utility function to find the maximum subarray
// sum by removing at most K array elements
int MaximumSubarraySum(int n, int* arr, int k)
{

    // dp[i][j]: Stores maximum subarray sum
    // by removing at most j elements from
    // subarray ending at i-th index
    int dp[n][k + 1];

    // Initialize dp[][] to 0
    memset(dp, 0, sizeof(dp));

    // Initialization for i = 0
    dp[0][0] = max(0, arr[0]);

    // Calculate maximum subarray sum by
    // removing at most j elements for
    // subarrays ending at i-th index
    for (int i = 1; i < n; i++) {
        for (int j = 0; j <= k; j++) {

            // Include current element in the subarray
            int X = max(0, arr[i] + dp[i - 1][j]);

            // Remove current element from the subarray
            int Y = (j > 0) ? dp[i - 1][j - 1] : 0;

            dp[i][j] = max(X, Y);
        }
    }

    // Stores maximum subarray sum by
    // removing at most K elements
    int res = 0;

    // Calculate maximum element
    // in dp[][]
    for (int i = 0; i < n; i++) {
        for (int j = 0; j <= k; j++) {

            // Update res
            res = max(res, dp[i][j]);
        }
    }

    // If all array elements are negative
    if (*max_element(arr, arr + n) < 0) {

        // Update res
        res = *max_element(arr, arr + n);
    }
    return res;
}

// Driver Code
int main()
{
    int arr[] = { -2, 1, 3, -2, 4, -7, 20 };
    int K = 1;
    int N = sizeof(arr) / sizeof(arr[0]);
    cout << MaximumSubarraySum(N, arr, K) << endl;

    return 0;
}
// this code is contributed by bhardwajji

Java

import java.util.Arrays;

class Main
{

  // Utility function to find the maximum subarray
  // sum by removing at most K array elements
  static int maximumSubarraySum(int n, int[] arr, int k) {

    // dp[i][j]: Stores maximum subarray sum
    // by removing at most j elements from
    // subarray ending at i-th index
    int[][] dp = new int[n][k + 1];

    // Initialize dp[][] to 0
    for (int i = 0; i < n; i++) {
      Arrays.fill(dp[i], 0);
    }

    // Initialization for i = 0
    dp[0][0] = Math.max(0, arr[0]);

    // Calculate maximum subarray sum by
    // removing at most j elements for
    // subarrays ending at i-th index
    for (int i = 1; i < n; i++) {
      for (int j = 0; j <= k; j++) {

        // Include current element in the subarray
        int X = Math.max(0, arr[i] + dp[i - 1][j]);

        // Remove current element from the subarray
        int Y = (j > 0) ? dp[i - 1][j - 1] : 0;

        dp[i][j] = Math.max(X, Y);
      }
    }

    // Stores maximum subarray sum by
    // removing at most K elements
    int res = 0;

    // Calculate maximum element
    // in dp[][]
    for (int i = 0; i < n; i++) {
      for (int j = 0; j <= k; j++) {

        // Update res
        res = Math.max(res, dp[i][j]);
      }
    }

    // If all array elements are negative
    if (Arrays.stream(arr).max().getAsInt() < 0) {

      // Update res
      res = Arrays.stream(arr).max().getAsInt();
    }
    return res;
  }

  // Driver Code
  public static void main(String[] args) {
    int[] arr = { -2, 1, 3, -2, 4, -7, 20 };
    int K = 1;
    int N = arr.length;
    System.out.println(maximumSubarraySum(N, arr, K));
  }
}

Python

# Python program to implement
# the above approach

# Utility function to find the maximum subarray
# sum by removing at most K array elements
def MaximumSubarraySum(n, arr, k):
    # dp[i][j]: Stores maximum subarray sum
    # by removing at most j elements from
    # subarray ending at i-th index
    dp = [[0 for j in range(k+1)] for i in range(n)]

    # Initialization for i = 0
    dp[0][0] = max(0, arr[0])

    # Calculate maximum subarray sum by
    # removing at most j elements for
    # subarrays ending at i-th index
    for i in range(1, n):
        for j in range(k+1):
            # Include current element in the subarray
            X = max(0, arr[i] + dp[i - 1][j])

            # Remove current element from the subarray
            Y = dp[i - 1][j - 1] if j > 0 else 0

            dp[i][j] = max(X, Y)

    # Stores maximum subarray sum by
    # removing at most K elements
    res = 0

    # Calculate maximum element in dp[][]
    for i in range(n):
        for j in range(k+1):
            # Update res
            res = max(res, dp[i][j])

    # If all array elements are negative
    if max(arr) < 0:
        # Update res
        res = max(arr)

    return res

# Driver Code
if __name__ == '__main__':
    arr = [-2, 1, 3, -2, 4, -7, 20]
    K = 1
    N = len(arr)
    print(MaximumSubarraySum(N, arr, K))

C#

using System;
using System.Linq;

class MainClass
{

  // Utility function to find the maximum subarray
  // sum by removing at most K array elements
  static int maximumSubarraySum(int n, int[] arr, int k)
  {

    // dp[i][j]: Stores maximum subarray sum
    // by removing at most j elements from
    // subarray ending at i-th index
    int[][] dp = new int[n][];

    // Initialize dp[][] to 0
    for (int i = 0; i < n; i++)
    {
      dp[i] = new int[k + 1];
      for (int j = 0; j <= k; j++)
      {
        dp[i][j] = 0;
      }
    }

    // Initialization for i = 0
    dp[0][0] = Math.Max(0, arr[0]);

    // Calculate maximum subarray sum by
    // removing at most j elements for
    // subarrays ending at i-th index
    for (int i = 1; i < n; i++)
    {
      for (int j = 0; j <= k; j++)
      {

        // Include current element in the subarray
        int X = Math.Max(0, arr[i] + dp[i - 1][j]);

        // Remove current element from the subarray
        int Y = (j > 0) ? dp[i - 1][j - 1] : 0;

        dp[i][j] = Math.Max(X, Y);
      }
    }

    // Stores maximum subarray sum by
    // removing at most K elements
    int res = 0;

    // Calculate maximum element
    // in dp[][]
    for (int i = 0; i < n; i++)
    {
      for (int j = 0; j <= k; j++)
      {

        // Update res
        res = Math.Max(res, dp[i][j]);
      }
    }

    // If all array elements are negative
    if (arr.Max() < 0)
    {

      // Update res
      res = arr.Max();
    }
    return res;
  }

  // Driver Code
  public static void Main()
  {
    int[] arr = { -2, 1, 3, -2, 4, -7, 20 };
    int K = 1;
    int N = arr.Length;
    Console.WriteLine(maximumSubarraySum(N, arr, K));
  }
}

Javascript

// JavaScript program to implement
// the above approach

// Utility function to find the maximum subarray
// sum by removing at most K array elements
function MaximumSubarraySum(n, arr, k) {

    // dp[i][j]: Stores maximum subarray sum
    // by removing at most j elements from
    // subarray ending at i-th index
    let dp = new Array(n);
    for (let i = 0; i < n; i++) {
        dp[i] = new Array(k + 1).fill(0);
    }

    // Initialization for i = 0
    dp[0][0] = Math.max(0, arr[0]);

    // Calculate maximum subarray sum by
    // removing at most j elements for
    // subarrays ending at i-th index
    for (let i = 1; i < n; i++) {
        for (let j = 0; j <= k; j++) {

            // Include current element in the subarray
            let X = Math.max(0, arr[i] + dp[i - 1][j]);

            // Remove current element from the subarray
            let Y = (j > 0) ? dp[i - 1][j - 1] : 0;

            dp[i][j] = Math.max(X, Y);
        }
    }

    // Stores maximum subarray sum by
    // removing at most K elements
    let res = 0;

    // Calculate maximum element
    // in dp[][]
    for (let i = 0; i < n; i++) {
        for (let j = 0; j <= k; j++) {

            // Update res
            res = Math.max(res, dp[i][j]);
        }
    }

    // If all array elements are negative
    if (Math.max(...arr) < 0) {

        // Update res
        res = Math.max(...arr);
    }
    return res;
}

// Driver Code
let arr = [-2, 1, 3, -2, 4, -7, 20];
let K = 1;
let N = arr.length;
console.log(MaximumSubarraySum(N, arr, K));

// This code is contributed by Prajwal Kandekar

Output:

26

Time Complexity: O(N * K)
Auxiliary Space: O(N * K)