Data Structures

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1. 01 Explain the difference between `map`, `forEach`, `filter`, and `reduce`. Easy
2. 02 Implement `Array.prototype.map` from scratch without using the native method. Medium
3. 03 Implement `Array.prototype.reduce` from scratch, including the initial-value edge case. Medium
4. 04 How does `Array.prototype.sort` work by default? Why is `[10, 2, 1].sort()` surprising? Medium
5. 05 Explain `flat` and `flatMap` and implement a recursive array flatten function for arbitrary depth. Medium
6. 06 What is the difference between mutating array methods (`push`, `splice`) and non-mutating ones (`slice`, `concat`)? Easy
7. 07 Implement a function to remove duplicates from an array while preserving order. Easy
8. 08 Explain stable vs unstable sorting algorithms and whether `Array.prototype.sort` is stable in modern engines. Hard
9. 09 How would you efficiently find the intersection of two large arrays? Medium
10. 10 Implement a debounced search-as-you-type array filter using `filter` and `includes`. Medium
11. 11 Explain array-like objects (e.g., `arguments`, NodeList) and how to convert them to real arrays. Medium
12. 12 Implement a sliding-window function to find the maximum sum subarray of size k. Medium
13. 13 How does `Array.isArray` differ from `typeof` and `instanceof` for detecting arrays? Easy
14. 14 Explain string immutability in JavaScript and its performance implications. Easy
15. 15 Implement a function to check if a string is a palindrome, ignoring case and punctuation. Easy
16. 16 Implement your own `String.prototype.trim` without using the native method. Easy
17. 17 How do template literals differ from string concatenation? Explain tagged templates. Medium
18. 18 Write a function to find the first non-repeating character in a string. Medium
19. 19 Explain Unicode, UTF-16 surrogate pairs, and why `'😀'.length` is 2, not 1. Hard
20. 20 Implement a function that counts the frequency of each character in a string. Easy
21. 21 Explain the difference between `String.raw`, escape sequences, and regular string literals. Medium
22. 22 Write an anagram-checking function and discuss its time complexity trade-offs (sort vs frequency map). Medium
23. 23 How would you implement a basic string compression algorithm (e.g., 'aabcccccaaa' -> 'a2b1c5a3')? Medium