✅ Sum of Subarray Minimums - LeetCode 907 - Monotonic Stack - Prefix Sum - Arrays - Explained

In this coding tutorial, we explore a Java solution to a common algorithmic problem. The task is to find the sum of the minimum elements within all contiguous subarrays of a given array. The solution involves efficient techniques using stacks and arrays.

🔍 Problem Statement:

Given an array of integers arr, we aim to calculate the sum of the minimum elements in every contiguous subarray. To handle potential large results, we return the answer modulo 10^9 + 7.

📝 Solution Overview:

We employ a stack-based approach to determine the indices of the previous and next less elements for each array element.
Using this information, we calculate the sum of minimum elements in all subarrays.
The solution is optimized for time complexity and ensures the answer remains within the specified modulo range.
👨‍💻 Code Walkthrough:

We dive into the Java implementation step by step, explaining key sections, such as stack manipulation, array initialization, and the modulo operation.

⚙️ Implementation Details:

Utilizing arrays for left and right indices.
Employing a stack to efficiently find previous and next less elements.
Applying the modulo operation to handle potential overflow.
🚨 Note:

Understanding this algorithm provides valuable insights into solving related problems and reinforces key data structure concepts.

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Let's dive into the code and enhance our algorithmic skills together! 💻📚
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[shubham]

What's the point on making video if you are just reading the solution. Try to give the intuition through which you came up with the solution. I do appreciate your efforts.

[shivambhanu]

I think in step 5, the last sub-point says the right array becomes [4, 2, 4, 4], but I think it's [1, 4, 4, 4].
I think It would be more helpful if you use an application like Microsoft Whiteboard to explain the intuition behind this approach while starting from a naive approach.
Learning using diagrams is faster and easier to grasp.
Happy Coding.

[shubhamgupta]

The value of right[] shown in the dry-run of your code is wrong. It should be [1, 4, 4, 4].

Anyhow Thanks for explaining the solution.

[sovietunion-ussr]

your voice is very loud i could hear every word you spoke in capslock .