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Maximum sum by picking elements from two arrays in order | Set 2
  • Last Updated : 08 Sep, 2020

Given two arrays A[] and B[], each of size N, and two integers X and Y denoting the maximum number of elements that can be picked from A[] and B[] respectively, the task is to find the maximum possible sum by selecting N elements in such a way that for any index i, either A[i] or B[i] can be chosen. 
Note: It is guaranteed that (X + Y) >= N.
Examples: 
 

Input: A[] = {1, 2, 3, 4, 5}, B[] = {5, 4, 3, 2, 1}, X = 3, Y = 2 
Output: 21 
i = 0 -> 5 picked 
i = 1 -> 4 picked 
i = 2 -> 3 picked 
i = 3 -> 4 picked 
i = 4 -> 5 picked 
5 + 4 + 3 + 4 + 5 = 21
Input: A[] = {1, 4, 3, 2, 7, 5, 9, 6}, B[] = {1, 2, 3, 6, 5, 4, 9, 8}, X = 4, Y = 4 
Output: 43 
 

Greedy Approach: The greedy approach to solve this problem has already been discussed in this post.
Dynamic Programming Approach: In this article, we are discussing a Dynamic Programming based solution. 
Follow the steps below to solve the problem: 
 

  1. Utmost min (N, X) elements can be selected from A[]and min(N, Y) elements can be selected from B[].
  2. Initialize a 2D array dp[][] such that dp[i][j] contains the maximum sum possible by selecting i elements from A[] and j elements from B[] where i and j ranges within min (N, X) and min (N, X) respectively.
  3. Initialize a variable max_sum to store the maximum sum possible.
  4. Traverse the array and for every array, element do the following: 
    • The (i + j)th element can either be A[i + j – 1] or B[i + j – 1].
    • If the (i + j)th element is selected from A[], then the cost of (i + 1)th element in A[] will be added to the total cost. Hence, the cost of selecting (i + 1)th element is dp[i][j] = dp[ i – 1 ][ j ] + A[ i + j – 1 ] for this case.
    • If the (i + j)th element is selected from B[], then the cost of selecting (i + 1)th element is dp[i][j] = dp[ i – 1 ][ j ] + B[ i + j – 1 ].
  5. Now, the target is to maximize the cost. Hence, the recurrence relation is: 
     

dp[i][j] = max(dp[ i – 1 ][ j ] + A[ i + j – 1 ], dp[ i – 1 ][ j ] + B[ i + j – 1 ]) 
 

  1. Keep updating max_sum after every iteration. After complete traversal of the array print the final value of max_sum.

Below is the implementation of the above approach : 
 



C++

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// C++ program to find maximum sum
// possible by selecting an element
// from one of two arrays for every index
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to calculate maximum sum
int maximumSum(int A[], int B[],
               int length,
               int X, int Y)
{
    int l = length;
    // Maximum elements that can be
    // chosen from array A
    int l1 = min(length, X);
 
    // Maximum elements that can be
    // chosen from array B
    int l2 = min(length, Y);
 
    int dp[l1 + 1][l2 + 1];
    memset(dp, 0, sizeof(dp));
    dp[0][0] = 0;
 
    // Stores the maximum
    // sum possible
    int max_sum = INT_MIN;
 
    // Fill the dp[][] for base case when
    // all elements are selected from A[]
    for (int i = 1; i <= l1; i++) {
        dp[i][0] = dp[i - 1][0] + A[i - 1];
        max_sum = max(max_sum, dp[i][0]);
    }
 
    // Fill the dp[][] for base case when
    // all elements are selected from B[]
    for (int i = 1; i <= l2; i++) {
        dp[0][i] = dp[0][i - 1] + B[i - 1];
        max_sum = max(max_sum, dp[0][i]);
    }
 
    for (int i = 1; i <= l1; i++) {
        for (int j = 1; j <= l2; j++) {
            if (i + j <= l)
                dp[i][j]
                    = max(dp[i - 1][j]
                              + A[i + j - 1],
                          dp[i][j - 1]
                              + B[i + j - 1]);
            max_sum = max(dp[i][j],
                          max_sum);
        }
    }
 
    // Return the final answer
    return max_sum;
}
 
// Driver Program
int main()
{
    int A[] = { 1, 2, 3, 4, 5 };
    int B[] = { 5, 4, 3, 2, 1 };
    int X = 3, Y = 2;
 
    int N = sizeof(A) / sizeof(A[0]);
 
    cout << maximumSum(A, B, N, X, Y);
 
    return 0;
}

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Java

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// Java program to find maximum sum
// possible by selecting an element
// from one of two arrays for every index
class GFG{
     
// Function to calculate maximum sum
static int maximumSum(int A[], int B[],
                      int length, int X,
                      int Y)
{
    int l = length;
     
    // Maximum elements that can be
    // chosen from array A
    int l1 = Math.min(length, X);
 
    // Maximum elements that can be
    // chosen from array B
    int l2 = Math.min(length, Y);
 
    int dp[][] = new int [l1 + 1][l2 + 1];
 
    // Stores the maximum
    // sum possible
    int max_sum = Integer.MIN_VALUE;
 
    // Fill the dp[][] for base case when
    // all elements are selected from A[]
    for(int i = 1; i <= l1; i++)
    {
        dp[i][0] = dp[i - 1][0] + A[i - 1];
        max_sum = Math.max(max_sum, dp[i][0]);
    }
 
    // Fill the dp[][] for base case when
    // all elements are selected from B[]
    for(int i = 1; i <= l2; i++)
    {
        dp[0][i] = dp[0][i - 1] + B[i - 1];
        max_sum = Math.max(max_sum, dp[0][i]);
    }
 
    for(int i = 1; i <= l1; i++)
    {
        for(int j = 1; j <= l2; j++)
        {
            if (i + j <= l)
                dp[i][j] = Math.max(dp[i - 1][j] +
                                     A[i + j - 1],
                                    dp[i][j - 1] +
                                     B[i + j - 1]);
            max_sum = Math.max(dp[i][j], max_sum);
        }
    }
     
    // Return the final answer
    return max_sum;
}
     
// Driver code
public static void main (String[] args)
{
    int A[] = new int[]{ 1, 2, 3, 4, 5 };
    int B[] = new int[]{ 5, 4, 3, 2, 1 };
     
    int X = 3, Y = 2;
    int N = A.length;
 
    System.out.println(maximumSum(A, B, N, X, Y));
}
}
 
// This code is contributed by Pratima Pandey

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Python

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# Python3 program to find maximum sum
# possible by selecting an element
# from one of two arrays for every index
# Function to calculate maximum sum
def maximumSum(A, B, length, X, Y):
    l = length
     
    # Maximum elements that can be
    # chosen from array A
    l1 = min(length, X)
 
    # Maximum elements that can be
    # chosen from array B
    l2 = min(length, Y)
 
    dp=[[ 0 for i in range(l2 + 1)]
        for i in range(l1 + 1)]
    dp[0][0] = 0
 
    # Stores the maximum
    # sum possible
    max_sum = -10 * 9
 
    # Fill the dp[]for base case when
    # all elements are selected from A[]
    for i in range(1, l1 + 1):
        dp[i][0] = dp[i - 1][0] + A[i - 1]
        max_sum = max(max_sum, dp[i][0])
 
 
    # Fill the dp[]for base case when
    # all elements are selected from B[]
    for i in range(1, l2 + 1):
        dp[0][i] = dp[0][i - 1] + B[i - 1]
        max_sum = max(max_sum, dp[0][i])
 
    for i in range(1, l1 + 1):
        for j in range(1, l2 + 1):
            if (i + j <= l):
                dp[i][j]= max(dp[i - 1][j] + A[i + j - 1],
                              dp[i][j - 1] + B[i + j - 1])
            max_sum = max(dp[i][j], max_sum)
 
    # Return the final answer
    return max_sum
 
# Driver Program
if __name__ == '__main__':
    A=  [1, 2, 3, 4, 5]
    B=  [5, 4, 3, 2, 1]
    X = 3
    Y = 2
    N = len(A)
    print(maximumSum(A, B, N, X, Y))
 
# This code is contributed by Mohit Kumar 29

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C#

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// C# program to find maximum sum
// possible by selecting an element
// from one of two arrays for every index
using System;
 
class GFG{
     
// Function to calculate maximum sum
static int maximumSum(int []A, int []B,
                      int length, int X,
                      int Y)
{
    int l = length;
     
    // Maximum elements that can be
    // chosen from array A
    int l1 = Math.Min(length, X);
 
    // Maximum elements that can be
    // chosen from array B
    int l2 = Math.Min(length, Y);
 
    int [,]dp = new int [l1 + 1, l2 + 1];
 
    // Stores the maximum
    // sum possible
    int max_sum = int.MinValue;
 
    // Fill the [,]dp for base case when
    // all elements are selected from []A
    for(int i = 1; i <= l1; i++)
    {
        dp[i, 0] = dp[i - 1, 0] + A[i - 1];
        max_sum = Math.Max(max_sum, dp[i, 0]);
    }
 
    // Fill the [,]dp for base case when
    // all elements are selected from []B
    for(int i = 1; i <= l2; i++)
    {
        dp[0, i] = dp[0, i - 1] + B[i - 1];
        max_sum = Math.Max(max_sum, dp[0, i]);
    }
 
    for(int i = 1; i <= l1; i++)
    {
        for(int j = 1; j <= l2; j++)
        {
            if (i + j <= l)
                dp[i, j] = Math.Max(dp[i - 1, j] +
                                     A[i + j - 1],
                                    dp[i, j - 1] +
                                     B[i + j - 1]);
            max_sum = Math.Max(dp[i, j], max_sum);
        }
    }
     
    // Return the readonly answer
    return max_sum;
}
     
// Driver code
public static void Main(String[] args)
{
    int []A = new int[]{ 1, 2, 3, 4, 5 };
    int []B = new int[]{ 5, 4, 3, 2, 1 };
     
    int X = 3, Y = 2;
    int N = A.Length;
 
    Console.WriteLine(maximumSum(A, B, N, X, Y));
}
}
 
// This code is contributed by Amit Katiyar

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Output: 

21


 

Time Complexity: O(N2) 
Auxiliary Space: O(N2)
 

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