String + Array Interview Questions list

 String-Based Questions

1. Reverse a string — without using built-ins.
2. Is it a palindrome? Let's find out.
3. Remove duplicates from a string — efficiently.
4. First non-repeating character — who stands alone?
5. Count how many times each character appears.
6. Flip the words in a sentence, not the letters.
7. Are two strings anagrams? Prove it.
8. Longest substring without repeats — sliding window style.
9. Build your own atoi — string to integer.
10. Compress strings with run-length encoding.
11. Most frequent character — who dominates?
12. List all possible substrings of a string.
13. Is one string a rotation of another?
14. Strip all white spaces from a string.
15. Is this a valid shuffle of two strings?
16. Convert text to Title Case — properly.
17. Find the longest common prefix among words.
18. Break a string into a char array — without confusion.
19. Replace spaces with %20 — classic URL trick.
20. Turn full sentences into acronyms.
21. Check if the string is all digits — no alphabets allowed.
22. Count how many words are in the string.
23. Remove a specific character — cleanly.
24. Find the shortest word in a sentence.
25. Longest palindromic substring — two-pointer style.

Array-Based Questions

26. Reverse an array in-place.
27. Find the largest and smallest element.
28. Check for duplicates in an array.
29. Remove duplicates — return only unique values.
30. Find the missing number from 1 to N.
31. Move all zeros to the end — keep order.
32. Rotate the array left/right by K positions.
33. Find the Kth largest/smallest element.
34. Merge two sorted arrays — without using extra space.
35. Find the intersection of two arrays.
36. Sort 0s, 1s, and 2s without using sort().
37. Find subarrays with a given sum.
38. Detect if a subarray sums to 0.
39. Find the longest increasing subsequence.
40. Kadane’s Algorithm — maximum subarray sum.
41. Check if the array is sorted and rotated.
42. Rearrange the array in max-min order alternately.
43. Find leaders in an array (no greater element to the right).
44. Calculate the frequency of all elements in O(n).
45. Product of all elements except self.