Python Solution for LeetCode 907 – Sum of Subarray Minimums using Monotonic Stack

In this video, we're solving LeetCode 907: Sum of Subarray Minimums using a powerful and optimized technique — the monotonic stack. 🚀

If you've ever struggled with subarray problems or wanted to understand how to efficiently calculate the contribution of each element in an array, this video is for you!

🔍 What You'll Learn:
What the problem "Sum of Subarray Minimums" is asking

Why brute force won't work (and how to avoid TLE)

How to use monotonic stacks to calculate "Previous Less Element" and "Next Less Element"

How to compute the final sum using element contributions

Clean and well-commented Python code you can use in your own practice

📊 Problem Overview:
Given an array of integers, you need to calculate the sum of the minimum values of all subarrays.
Instead of checking all subarrays (which takes O(n²) or O(n³) time), we’ll use a smart linear-time approach with stacks.

⏱️ Timestamps:
00:00 – Intro
00:10 – Problem Explanation
00:18 – Example
00:41 – Bruth Force
00:55 –monotonic Stack
01:00 – Clean Trick
01:12 – Solution
01:24– Time & Space Complexity
01:35 – Final Thoughts

💡 Solution Summary:
Each element in the array can act as the minimum for multiple subarrays. Instead of generating each subarray, we:
Calculate how many subarrays each element is the minimum of
Use two passes with a monotonic stack:
One for Previous Less Elements
One for Next Less Elements

Multiply the count of left and right contributions for each element
Multiply by the element's value to get its total contribution
Sum everything up, modulo 10^9 + 7

🧠 Ideal For:
Coding interview prep
Practicing dynamic programming and stack problems
Python developers preparing for FAANG-style interviews
Anyone following a 30-day coding challenge