fix(guide): simplify directory structure

guide/english/certifications/coding-interview-prep/data-structures/learn-how-a-stack-works/index.md

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+---
+title: Learn how a Stack Works
+---
+## Learn how a Stack Works
+
+### Method:
+- Stacks are an abstract data structures.
+- They follow LIFO (Last In First Out) or FILO (First In Last Out) principle.
+- Stack's insertion and deletion operations are of **O(1)** time complexity.
+- In Javascript, arrays can be treated as a Stack since `.push()` and `.pop()` methods have time complexity of **O(1)**.
+- In this challenge we need to `.pop()` and then `.push()` into the stack.
+
+### Solution:
+```js
+var homeworkStack = ["BIO12","HIS80","MAT122","PSY44"];
+
+homeworkStack.pop();
+homeworkStack.push("CS50");
+```
+### Reference:
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Stack_(abstract_data_type))
+- Video by [Hackerrank](https://www.youtube.com/watch?v=wjI1WNcIntg)