This content originally appeared on DEV Community and was authored by DRASHTI BUDDHADEV
There are few things that initially did not make sense to me- What first seemed confusing and senseless later turned out to be brilliantly logical.
In this article we'll go through some "simple" yet mind-bending stack behaviors that will make you question everything!
Let's start with an easy one:
let stack = [];
stack.push("first");
stack.push("second");
console.log(stack[0]);
Obviously, it prints "second" because stacks are LIFO, right?
Correct Answer: "first"
WAIT, WHAT?!
Why?? JavaScript arrays used as stacks still maintain their array indexing! Index 0 is always the first element you pushed, not the "top" of your logical stack. The stack behavior only applies to push() and pop() methods, not array access !
> Interesting, right? Let’s move on to the 2nd:
let s1 = new Stack();
let s2 = new Stack();
s1.push(1); s1.push(2);
s2.push(1); s2.push(2);
console.log(s1 == s2);
Guessed the answer?
Correct Answer is: false
Why? Stacks are objects → compared by reference, not content.
Even identical content → different memory → false.
Want equality? Implement equals() manually.
> Okay, next one is my favorite!
If you are someone preparing for interviews or are someone like me who has started making DSA a routine, here's what you must know:
A monotonic stack is a savior for problems where:
-Boundaries* or ranges are to be found for each element of an array.
-The previous smallest element and the next smallest element are of concern for each element of an array.It’s called “monotonic” because the elements in the stack always maintain a certain order — either increasing or decreasing.
-A monotonic increasing stack keeps elements in ascending order (used to find the next smaller element).
-A monotonic decreasing stack keeps elements in descending order (used to find the next greater element).
Why it’s powerful:
It turns problems that seem like they require nested loops (O(n²)) into linear O(n) solutions. You traverse the array just once, pushing and popping elements intelligently.
Quick pattern recap:(of problems where stack would be used)
- Move through the array (usually from right to left).
- While the stack is not empty and the top element violates your “monotonic” condition, pop it.
- The element on top (after popping) is your next smaller or greater one.
- Push the current element into the stack.
_**> What I’ve learned:
The monotonic stack isn’t just a trick — it’s a mindset. Once you see the pattern behind these problems, you start recognizing it everywhere.**_
Since I just mentioned the pattern behind the problems — you can’t miss what’s coming next - these classic problems will blow your mind!
Note: Just because you understand the pattern doesn’t mean you’ll code it perfectly on the first go — it took me multiple submissions (and a few “why isn’t this working!?” moments!). That’s how you really learn. Keep going, it clicks eventually!
- Next Greater Element
- Sum of Subarray Minimums
- Sum of Subarray Ranges
- Trapping Rain Water
- Largest Rectangle in Histogram
Which part of stacks changed how you think about problem-solving? I’d love to hear your take in the comments.
_Thanks for reading till the end — I hope this helped you connect a few dots and made your DSA journey a little brighter.
And if there’s any insight or trick about stacks that I might’ve missed, drop it in the comments too — I’d love to learn from you as well!
This content originally appeared on DEV Community and was authored by DRASHTI BUDDHADEV
DRASHTI BUDDHADEV | Sciencx (2025-10-29T17:16:01+00:00) Once You See the Pattern, You Can’t Unsee It — The Stack Story. Retrieved from https://www.scien.cx/2025/10/29/once-you-see-the-pattern-you-cant-unsee-it-the-stack-story/
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