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Maximize sum of K elements selected from a Matrix such that each selected element must be preceded by selected row elements
  • Last Updated : 07 Dec, 2020

Given a 2D array arr[][] of size N * M, and an integer K, the task is to select K elements with maximum possible sum such that if an element arr[i][j] is selected, then all the elements from the ith row present before the jth column needs to be selected.

Examples:

Input: arr[][] = {{10, 10, 100, 30}, {80, 50, 10, 50}}, K = 5
Output: 250
Explanation:
Selecting first 3 elements from the first row, sum = 10 + 10 + 100 = 120
Selecting first 2 elements from the second row, sum = 80 + 50 = 130
Therefore, the maximum sum = 120 + 130 = 250.

Input: arr[][] = {{30, 10, 110, 13}, {810, 152, 12, 5}, {124, 24, 54, 124}}, K = 6
Output: 1288
Explanation:
Selecting first 2 elements from the second row, sum = 810 + 152 = 962
Selecting all 4 elements from the third row, sum = 124 + 24 + 54 + 124 = 326
Therefore, the maximum sum = 962 + 326 = 1288

 

Naive Approach: The simplest approach to solve the problem is to generate all possible subsets of size K from the given matrix based on specified constraint and calculate the maximum sum possible from these subsets. 



Time Complexity: O((N*M)!/(K!)*(N*M – K)!)
Auxiliary Space: O(1)

Efficient Approach: The above approach can be optimized by using Dynamic Programming. The idea is to find the maximum sum of selecting i elements till the jth row of the 2D array arr[][]. The auxiliary array dp[i][j + 1] stores the maximum sum of selecting i elements till jth row of matrix arr[][]. Follow the steps below to solve the problem:

  • Initialize a dp[][] table of size (K+1)*(N+1) and initialize dp[0][i] as 0, as no elements have sum equal to 0.
  • Initialize dp[i][0] as 0, as there are no elements to be selected.
  • Iterate over the range [0, K] using two nested loops and [0, N] using the variables i and j respectively:
    • Initialize a variable, say sum as 0, to keep track of the cumulative sum of elements in the jth row of arr[][].
    • Initialize maxSum as dp[i][j], if no item needs to be selected from jth row of arr[][].
    • Iterate with k as 1 until k > M or k > i:
      • Increment sum += buy[j][k – 1].
      • Store the maximum of existing maxSum and (sum + dp[i – k][j]) in maxSum.
    • Store dp[i][j + 1] as the value of maxSum.
  • Print the value dp[K][N] as the maximum sum of K elements till (N – 1)th row of matrix arr[][].

Below is the implementation of the above approach:

C++

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// C++ program for the above approach
#include <iostream>
using namespace std;
 
// Function to return the maximum
// of two elements
int max(int a, int b)
{
    return a > b ? a : b;
}
 
// Function to find the maximum sum
// of selecting K elements from
// the given 2D array arr[][]
int maximumsum(int arr[][4], int K,
               int N, int M)
{
    int sum = 0, maxSum;
    int i, j, k;
 
    // dp table of size (K+1)*(N+1)
    int dp[K + 1][N + 1];
 
    // Initialize dp[0][i] = 0
    for (i = 0; i <= N; i++)
        dp[0][i] = 0;
 
    // Initialize dp[i][0] = 0
    for (i = 0; i <= K; i++)
        dp[i][0] = 0;
 
    // Selecting i elements
    for (i = 1; i <= K; i++) {
 
        // Select i elements till jth row
        for (j = 0; j < N; j++) {
 
            // dp[i][j+1] is the maximum
            // of selecting i elements till
            // jth row
 
            // sum = 0, to keep track of
            // cummulative elements sum
            sum = 0;
            maxSum = dp[i][j];
 
            // Traverse arr[j][k] until
            // number of elements until k>i
            for (k = 1; k <= M && k <= i; k++) {
 
                // Select arr[j][k - 1]th item
                sum += arr[j][k - 1];
 
                maxSum
                    = max(maxSum,
                          sum + dp[i - k][j]);
            }
 
            // Store the maxSum in dp[i][j+1]
            dp[i][j + 1] = maxSum;
        }
    }
 
    // Return the maximum sum
    return dp[K][N];
}
 
// Driver Code
int main()
{
    int arr[][4] = { { 10, 10, 100, 30 },
                     { 80, 50, 10, 50 } };
 
    int N = 2, M = 4;
    int K = 5;
 
    // Function Call
    cout << maximumsum(arr, K, N, M);
 
    return 0;
}

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Java

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// Java program for the above approach
import java.util.*;
    
class GFG{
    
// Function to return the maximum
// of two elements
static int max(int a, int b)
{
    return a > b ? a : b;
}
  
// Function to find the maximum sum
// of selecting K elements from
// the given 2D array arr[][]
static int maximumsum(int arr[][], int K,
                      int N, int M)
{
    int sum = 0, maxSum;
    int i, j, k;
  
    // dp table of size (K+1)*(N+1)
    int[][] dp = new int[K + 1][N + 1];
  
    // Initialize dp[0][i] = 0
    for(i = 0; i <= N; i++)
        dp[0][i] = 0;
  
    // Initialize dp[i][0] = 0
    for(i = 0; i <= K; i++)
        dp[i][0] = 0;
  
    // Selecting i elements
    for(i = 1; i <= K; i++)
    {
         
        // Select i elements till jth row
        for(j = 0; j < N; j++)
        {
             
            // dp[i][j+1] is the maximum
            // of selecting i elements till
            // jth row
  
            // sum = 0, to keep track of
            // cummulative elements sum
            sum = 0;
            maxSum = dp[i][j];
  
            // Traverse arr[j][k] until
            // number of elements until k>i
            for(k = 1; k <= M && k <= i; k++)
            {
                 
                // Select arr[j][k - 1]th item
                sum += arr[j][k - 1];
  
                maxSum = Math.max(maxSum,
                                  sum + dp[i - k][j]);
            }
  
            // Store the maxSum in dp[i][j+1]
            dp[i][j + 1] = maxSum;
        }
    }
  
    // Return the maximum sum
    return dp[K][N];
}
    
// Driver Code
public static void main(String[] args)
{
    int arr[][] =  { { 10, 10, 100, 30 },
                     { 80, 50, 10, 50 } };
  
    int N = 2, M = 4;
    int K = 5;
  
    // Function Call
    System.out.print(maximumsum(arr, K, N, M));
}
}
 
// This code is contributed by susmitakundugoaldanga

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Python3

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# Python program for the above approach
import math;
 
# Function to return the maximum
# of two elements
def max(a, b):
    if(a > b):
        return a;
    else:
        return b;
 
# Function to find the maximum sum
# of selecting K elements from
# the given 2D array arr
def maximumsum(arr, K, N, M):
    sum = 0;
    maxSum = 0;
 
    # dp table of size (K+1)*(N+1)
    dp = [[0 for i in range(N + 1)] for j in range(K + 1)]
 
    # Initialize dp[0][i] = 0
    for i in range(0, N + 1):
        dp[0][i] = 0;
 
    # Initialize dp[i][0] = 0
    for i in range(0, K + 1):
        dp[i][0] = 0;
 
    # Selecting i elements
    for  i in range(1, K + 1):
 
        # Select i elements till jth row
        for  j in range(0, N):
 
            # dp[i][j+1] is the maximum
            # of selecting i elements till
            # jth row
 
            # sum = 0, to keep track of
            # cummulative elements sum
            sum = 0;
            maxSum = dp[i][j];
 
            # Traverse arr[j][k] until
            # number of elements until k>i
            for k in range(1, i + 1):
                if(k > M):
                    break;
                     
                # Select arr[j][k - 1]th item
                sum += arr[j][k - 1];
 
                maxSum = max(maxSum, sum + dp[i - k][j]);
 
            # Store the maxSum in dp[i][j+1]
            dp[i][j + 1] = maxSum;
 
    # Return the maximum sum
    return dp[K][N];
 
# Driver Code
if __name__ == '__main__':
    arr = [[10, 10, 100, 30], [80, 50, 10, 50]];
 
    N = 2;
    M = 4;
    K = 5;
 
    # Function Call
    print(maximumsum(arr, K, N, M));
 
 
    # This code is contributed by 29AjayKumar

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C#

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// C# program for the above approach
using System;
 
class GFG{
    
// Function to return the maximum
// of two elements
static int max(int a, int b)
{
    return a > b ? a : b;
}
  
// Function to find the maximum sum
// of selecting K elements from
// the given 2D array [,]arr
static int maximumsum(int [,]arr, int K,
                      int N, int M)
{
    int sum = 0, maxSum;
    int i, j, k;
     
    // dp table of size (K+1)*(N+1)
    int[,] dp = new int[K + 1, N + 1];
  
    // Initialize dp[0,i] = 0
    for(i = 0; i <= N; i++)
        dp[0, i] = 0;
  
    // Initialize dp[i,0] = 0
    for(i = 0; i <= K; i++)
        dp[i, 0] = 0;
  
    // Selecting i elements
    for(i = 1; i <= K; i++)
    {
         
        // Select i elements till jth row
        for(j = 0; j < N; j++)
        {
             
            // dp[i,j+1] is the maximum
            // of selecting i elements till
            // jth row
  
            // sum = 0, to keep track of
            // cummulative elements sum
            sum = 0;
            maxSum = dp[i, j];
  
            // Traverse arr[j,k] until
            // number of elements until k>i
            for(k = 1; k <= M && k <= i; k++)
            {
                 
                // Select arr[j,k - 1]th item
                sum += arr[j, k - 1];
  
                maxSum = Math.Max(maxSum,
                                  sum + dp[i - k, j]);
            }
  
            // Store the maxSum in dp[i,j+1]
            dp[i, j + 1] = maxSum;
        }
    }
  
    // Return the maximum sum
    return dp[K, N];
}
    
// Driver Code
public static void Main(String[] args)
{
    int [,]arr =  { { 10, 10, 100, 30 },
                    { 80, 50, 10, 50 } };
  
    int N = 2, M = 4;
    int K = 5;
  
    // Function Call
    Console.Write(maximumsum(arr, K, N, M));
}
}
 
// This code is contributed by Amit Katiyar

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Output: 

250

 

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

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