Maximize product of subarray sum with its minimum element

Given an array arr[] consisting of N positive integers, the task is to find the maximum product of subarray sum with the minimum element of that subarray.

Examples:

Input: arr[] = {3, 1, 6, 4, 5, 2}
Output: 60
Explanation:
The required maximum product can be obtained using subarray {6, 4, 5}
Therefore, maximum product = (6 + 4 + 5) * (4) = 60

Input: arr[] = {4, 1, 2, 9, 3}
Output: 81
Explanation:
The required maximum product can be obtained using subarray {9}
Maximum product = (9)* (9) = 81

Naive Approach: The simplest approach to solve the problem is to generate all subarrays of the given array and for each subarray, calculate the sum of the subarray, and multiply it with the minimum element in the subarray. Update the maximum product by comparing it with the product calculated. Finally, print maximum product obtained after processing all the subarray.



 Time Complexity: O(N3)
Auxiliary Space: O(1)

Efficient Approach: The above approach can be optimized using a Stack and Prefix Sum Array. The idea is to use the stack to get the index of nearest smaller elements on the left and right of each element. Now, using these, the required product can be obtained. Follow the steps below to solve the problem:

  • Initialize an array presum[] to store all the resultant prefix sum array of the given array.
  • Initialize two arrays l[] and r[] to store the index of the nearest left and right smaller elements respectively.
  • For every element arr[i], calculate l[i] and r[i] using a stack.
  • Traverse the given array and for each index i, the product can be calculated by:

arr[i] * (presum[r[i]] – presum[l[i]-1])

  • Print the maximum product after completing all the above steps

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 find the
// maximum product possible
void maxValue(int a[], int n)
{
     
    // Stores prefix sum
    int presum[n];
 
    presum[0] = a[0];
 
    // Find the prefix sum array
    for(int i = 1; i < n; i++)
    {
        presum[i] = presum[i - 1] + a[i];
    }
 
    // l[] and r[] stores index of
    // nearest smaller elements on
    // left and right respectively
    int l[n], r[n];
 
    stack<int> st;
 
    // Find all left index
    for(int i = 1; i < n; i++)
    {
         
        // Until stack is non-empty
        // & top element is greater
        // than the current element
        while (!st.empty() &&
              a[st.top()] >= a[i])
            st.pop();
 
        // If stack is empty
        if (!st.empty())
            l[i] = st.top() + 1;
        else
            l[i] = 0;
 
        // Push the current index i
        st.push(i);
    }
 
    // Reset stack
    while(!st.empty())
    st.pop();
 
    // Find all right index
    for(int i = n - 1; i >= 0; i--)
    {
         
        // Until stack is non-empty
        // & top element is greater
        // than the current element
        while (!st.empty() &&
              a[st.top()] >= a[i])
            st.pop();
 
            if (!st.empty())
                r[i] = st.top() - 1;
            else
                r[i] = n - 1;
 
        // Push the current index i
        st.push(i);
    }
 
    // Stores the maximum product
    int maxProduct = 0;
 
    int tempProduct;
 
    // Iterate over the range [0, n)
    for(int i = 0; i < n; i++)
    {
         
        // Calculate the product
        tempProduct = a[i] * (presum[r[i]] -
                     (l[i] == 0 ? 0 :
                    presum[l[i] - 1]));
 
        // Update the maximum product
        maxProduct = max(maxProduct,
                        tempProduct);
    }
 
    // Return the maximum product
    cout << maxProduct;
}
 
// Driver Code
int main()
{
     
    // Given array
    int n = 6;
    int arr[] = { 3, 1, 6, 4, 5, 2 };
 
    // Function call
    maxValue(arr, n);
}
 
// This code is contributed by grand_master

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Java

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// Java program to implement
// the above approach
 
import java.util.*;
 
class GFG {
 
    // Function to find the
    // maximum product possible
    public static void
    maxValue(int[] a, int n)
    {
 
        // Stores prefix sum
        int[] presum = new int[n];
 
        presum[0] = a[0];
 
        // Find the prefix sum array
        for (int i = 1; i < n; i++) {
 
            presum[i] = presum[i - 1] + a[i];
        }
 
        // l[] and r[] stores index of
        // nearest smaller elements on
        // left and right respectively
        int[] l = new int[n], r = new int[n];
 
        Stack<Integer> st = new Stack<>();
 
        // Find all left index
        for (int i = 1; i < n; i++) {
 
            // Until stack is non-empty
            // & top element is greater
            // than the current element
            while (!st.isEmpty()
                   && a[st.peek()] >= a[i])
                st.pop();
 
            // If stack is empty
            if (!st.isEmpty())
                l[i] = st.peek() + 1;
            else
                l[i] = 0;
 
            // Push the current index i
            st.push(i);
        }
 
        // Reset stack
        st.clear();
 
        // Find all right index
        for (int i = n - 1; i >= 0; i--) {
 
            // Until stack is non-empty
            // & top element is greater
            // than the current element
            while (!st.isEmpty()
                   && a[st.peek()] >= a[i])
                st.pop();
 
            if (!st.isEmpty())
                r[i] = st.peek() - 1;
            else
                r[i] = n - 1;
 
            // Push the current index i
            st.push(i);
        }
 
        // Stores the maximum product
        int maxProduct = 0;
 
        int tempProduct;
 
        // Iterate over the range [0, n)
        for (int i = 0; i < n; i++) {
 
            // Calculate the product
            tempProduct
                = a[i]
                  * (presum[r[i]]
                     - (l[i] == 0 ? 0
                                  : presum[l[i] - 1]));
 
            // Update the maximum product
            maxProduct
                = Math.max(maxProduct,
                           tempProduct);
        }
 
        // Return the maximum product
        System.out.println(maxProduct);
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        // Given array
        int[] arr = { 3, 1, 6, 4, 5, 2 };
 
        // Function Call
        maxValue(arr, arr.length);
    }
}

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C#

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// C# program to implement
// the above approach
using System;
using System.Collections.Generic;
 
class GFG{
     
// Function to find the
// maximum product possible
public static void maxValue(int[] a,
                            int n)
{
     
    // Stores prefix sum
    int[] presum = new int[n];
 
    presum[0] = a[0];
 
    // Find the prefix sum array
    for(int i = 1; i < n; i++)
    {
        presum[i] = presum[i - 1] + a[i];
    }
 
    // l[] and r[] stores index of
    // nearest smaller elements on
    // left and right respectively
    int[] l = new int[n], r = new int[n];
     
    Stack<int> st = new Stack<int>();
 
    // Find all left index
    for(int i = 1; i < n; i++)
    {
         
        // Until stack is non-empty
        // & top element is greater
        // than the current element
        while (st.Count > 0 &&
           a[st.Peek()] >= a[i])
            st.Pop();
 
        // If stack is empty
        if (st.Count > 0)
            l[i] = st.Peek() + 1;
        else
            l[i] = 0;
 
        // Push the current index i
        st.Push(i);
    }
 
    // Reset stack
    st.Clear();
 
    // Find all right index
    for(int i = n - 1; i >= 0; i--)
    {
         
        // Until stack is non-empty
        // & top element is greater
        // than the current element
        while (st.Count > 0 &&
           a[st.Peek()] >= a[i])
            st.Pop();
 
        if (st.Count > 0)
            r[i] = st.Peek() - 1;
        else
            r[i] = n - 1;
 
        // Push the current index i
        st.Push(i);
    }
 
    // Stores the maximum product
    int maxProduct = 0;
 
    int tempProduct;
 
    // Iterate over the range [0, n)
    for(int i = 0; i < n; i++)
    {
         
        // Calculate the product
        tempProduct = a[i] * (presum[r[i]] -
                     (l[i] == 0 ? 0 :
                     presum[l[i] - 1]));
 
        // Update the maximum product
        maxProduct = Math.Max(maxProduct,
                             tempProduct);
    }
 
    // Return the maximum product
    Console.WriteLine(maxProduct);
}
 
// Driver code
static void Main()
{
     
    // Given array
    int[] arr = { 3, 1, 6, 4, 5, 2 };
     
    // Function call
    maxValue(arr, arr.Length);
}
}
 
// This code is contributed by divyeshrabadiya07

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Output: 

60



 

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

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