# Maximum product of indexes of next greater on left and right

## Introduction

In the realm of computer science and algorithmic problem-solving, there are numerous challenges that require innovative thinking and efficient solutions. One such intriguing problem is finding the maximum product of indexes of the next greater elements on the left and right for each element in an array. This problem not only tests one's ability to traverse and manipulate arrays but also highlights the importance of algorithmic optimization.

## Problem Statement

Given an array of integers, let's call it 'arr,' we are tasked with finding the maximum possible product of the index of the next greater element on the left and the index of the next greater element on the right for each element in the array. More formally, for each element 'arr[i],' we want to find 'left[i]' and 'right[i]' such that:

• 'left[i]' is the index of the nearest element on the left of 'arr[i]' which is greater than 'arr[i].'
• 'right[i]' is the index of the nearest element on the right of 'arr[i]' which is greater than 'arr[i].'

Our goal is to maximize the product 'left[i] * right[i]' for all 'i' in the array 'arr.'

For example,

consider the array: [4, 3, 2, 1, 5, 6]. The maximum product for each element would be: [5, 5, 5, 5, 6, 6], as illustrated below:

• For 4, the next greater element on the left and right is 5, so the product is 5*5 = 25.
• For 3, the next greater element on the left and right is 5, so the product is 5*5 = 25.
• For 2, the next greater element on the left and right is 5, so the product is 5*5 = 25.
• For 1, the next greater element on the left and right is 5, so the product is 5*5 = 25.
• For 5, the next greater element on the left is 6, and on the right is 6, so the product is 6*6 = 36.
• For 6, the next greater element on the left is 6, and on the right is 6, so the product is 6*6 = 36.

## Efficient Algorithm:

To solve this problem efficiently, we can use a stack-based approach. We'll maintain two stacks, one to store the indices of elements on the left side and another for the right side while iterating through the array. The key idea is to find the next greater element to the left and right for each element and calculate the product of their indices.

Here is the step-by-step algorithm:

• Initialize two empty stacks, leftStack and rightStack.
• Initialize two arrays, leftGreater and rightGreater, to store the indices of the next greater elements on the left and right for each element.
• Iterate through the array from left to right:
1. For each element arr[i], while the leftStack is not empty and arr[i] > arr[leftStack.top()], pop elements from leftStack and set leftGreater[leftStack.pop()] = i.
• Iterate through the array from right to left:
1. For each element arr[i], while the rightStack is not empty and arr[i] > arr[rightStack.top()], pop elements from rightStack and set rightGreater[rightStack.pop()] = i.
• Calculate the maximum product for each element:
1. Iterate through the array and find the maximum product as max(leftGreater[i] * rightGreater[i]).
• Return the result.

## Implementation:

Explanation:

• The primary function in the program is getMaxProductIndexes, which accepts a reference to a vector of integers as input and returns another vector of integers as output.
• Within this function, several key variables are initialized, such as n, which stores the size of the input vector. Two arrays, leftGreater and rightGreater, are also created and initialized with -1 values.
• Additionally, two stack data structures, leftStack and rightStack, are set up to facilitate intermediate calculations.
• The program proceeds to compute the leftGreater array, which stores the indices of the next greater elements to the left of each element in the input vector.
• To do this, it employs the leftStack to maintain a non-increasing order of elements while simultaneously updating the leftGreater array.
• Following this, the program calculates the rightGreater array, responsible for storing the indices of the next greater elements to the right of each element in the input vector.
• This is accomplished by traversing the input vector in reverse order and utilizing the rightStack in a similar manner to the leftStack.
• After obtaining both the leftGreater and rightGreater arrays, the program proceeds to determine the maximum product of indexes for each element in the input vector.
• It initializes a vector called maxProductIndexes to hold these products and another variable, maxProduct, to track the maximum product seen thus far.
• The product for each element is computed as (leftGreater[i] + 1) * (rightGreater[i] + 1), wherein a +1 is added to the indices to switch from 0-based indexing to 1-based indexing.

Program Output:

### Time and Space Complexity Analysis:

Understanding the time and space complexity of the algorithm is crucial, especially when dealing with larger input arrays.

Time Complexity:

The algorithm traverses the input array twice, once from left to right and once from right to left. Each traversal takes O(N) time, where N is the number of elements in the array.

In each traversal, elements are pushed and popped from the stacks, but each element is processed only once. So, the overall time complexity is O(N).

Space Complexity:

The space complexity is primarily determined by the space used by the two stacks (leftStack and rightStack). In the worst case, both stacks can have all N elements.

Therefore, the space complexity is O(N).

## Significance

This problem has practical applications in various domains, including data analysis and optimization. For instance, in finance, it can help identify critical points in a time series dataset, where one might want to buy or sell assets. In computer science, it is relevant for optimizing data structures and algorithms. Additionally, it serves as an excellent exercise for algorithmic problem-solving and can improve one's skills in dynamic programming and array manipulation.

## Efficiency and Use Cases:

The efficiency of this algorithm is notable because it optimally solves the problem without the need for nested loops or excessive iterations. This makes it suitable for real-world scenarios where quick computations are required.

Use cases for solving the "Maximum Product of Indexes of Next Greater on Left and Right" problem include:

• Stock Trading Strategies: Traders can use this algorithm to analyze historical stock price data to find the optimal buy and sell points based on the maximum product of index differences.
• Data Analysis: Data analysts may employ this algorithm to identify intervals with the greatest change in certain metrics, aiding in decision-making and trend analysis.
• Natural Language Processing (NLP): In NLP, this algorithm could be applied to analyze text data, identifying the most significant differences between two or more documents, such as articles or essays.
• Image Processing: In image processing, it can be used to find regions of interest or to calculate the most significant differences between two images, which can be useful in various applications like computer vision.

## Conclusion:

In conclusion, the "Maximum Product of Indexes of Next Greater Elements on the Left and Right" problem is a fascinating challenge that tests one's algorithmic skills and understanding of data structures, particularly monotonic stacks. This problem requires us to find the maximum possible product of indexes, representing the positions of the next greater elements to the left and right for each element in an array.

Through the use of two monotonic stacks, one for finding the next greater element on the left and another for the right, we can efficiently solve this problem in linear time complexity.

The key steps in solving the problem include iterating through the array while maintaining these stacks, populating the 'left' and 'right' arrays, and finally calculating the maximum product of indexes. The algorithm is not only elegant but also showcases the importance of algorithmic optimization in solving real-world problems efficiently.

By understanding and implementing this algorithm, programmers can gain valuable insights into how to tackle similar array-related problems, making it a valuable addition to their problem-solving toolkit.

The "Maximum Product of Indexes of Next Greater Elements on the Left and Right" problem serves as a great example of the intersection of data structures, algorithms, and creativity in solving complex computational challenges.