Top 50 Most Frequently Asked String Coding Problems for Interviews

Strings are one of the most common topics in coding interviews because they combine data structure knowledge, algorithmic thinking and real-world scenarios (search, validation, parsing, compression).

The key to cracking them? Pattern recognition. Most problems fall into common categories like two-pointers, sliding window, hashing, dynamic programming or backtracking.

This blog lists the Top 50 String Problems with clear descriptions + topic hints so you know exactly what the problem is and which approach to apply.

Use this as your study roadmap + tracking checklist .

1–10: String Basics & Manipulation

1. Reverse a String – Given a string, reverse its characters (without using built-in reverse).
    Topic Hint: Two-pointer or stack basics.

2. Check Palindrome String – Verify if a string reads the same forward and backward (e.g., “madam”).
    Topic Hint: Two-pointer comparison.

3. Find First Non-Repeating Character – Return the first character that doesn’t repeat in the string.
    Topic Hint: Hash map frequency counting.

4. Count Vowels & Consonants – Count how many vowels and consonants exist in the string.
    Topic Hint: Iteration + character classification.

5. Anagram Check – Check if two strings are rearrangements of each other.
    Topic Hint: Sorting or hash map frequency.

6. Group Anagrams – Group strings that are anagrams of each other.
    Topic Hint: Hash map with sorted signature or frequency map.

7. Longest Common Prefix – Find the longest shared prefix among a list of strings.
    Topic Hint: Sorting + prefix checking.

8. Remove Duplicates from String – Remove duplicate characters (e.g., “banana” → “ban”).
    Topic Hint: Hash set or stack.

9. Check Rotation of Another String – Check if one string is a rotation of another.
    Topic Hint: Concatenation + substring search.

10. String Compression – Compress repeated characters (e.g., “aaabb” → “a3b2”).
     Topic Hint: Run-length encoding logic.

11–20: Substring & Subsequence Problems

11. Longest Substring Without Repeating Characters – Find the longest substring with all unique characters.
     Topic Hint: Sliding window + hash map.

12. Minimum Window Substring – Smallest substring containing all characters of a target string.
     Topic Hint: Sliding window with frequency maps.

13. Longest Palindromic Substring – Find the longest palindromic substring.
     Topic Hint: Expand-around-center or DP.

14. Count All Palindromic Substrings – Count all palindromic substrings in a string.
     Topic Hint: DP or center expansion.

15. Longest Common Subsequence – Longest subsequence present in both strings.
     Topic Hint: Classic DP.

16. Longest Repeating Subsequence – Longest subsequence appearing more than once.
     Topic Hint: DP with index constraints.

17. Edit Distance (Levenshtein) – Minimum operations to convert one string to another.
     Topic Hint: DP (insertion, deletion, replacement).

18. Check Subsequence – Check if one string is a subsequence of another.
     Topic Hint: Two-pointer traversal.

19. Smallest Window Containing All Characters – Find the shortest substring containing all unique characters.
     Topic Hint: Sliding window technique.

20. Find All Subsequences of a String – Generate all subsequences.
     Topic Hint: Recursion/backtracking.

21–30: Pattern Matching & Searching

21. Rabin-Karp Algorithm – Search for a substring using rolling hash.
     Topic Hint: Hashing + sliding window.

22. KMP Algorithm – Pattern matching using prefix function.
     Topic Hint: Prefix table preprocessing.

23. Z-Algorithm for String Matching – Preprocess Z-array for fast substring search.
     Topic Hint: Linear preprocessing technique.

24. Wildcard Matching – Match string against ? and *.
     Topic Hint: DP on pattern and string.

25. Regular Expression Matching – Match with regex symbols . and *.
     Topic Hint: Hard-level DP.

26. Implement strstr() – Implement substring search function manually.
     Topic Hint: Naïve search or KMP.

27. Count Pattern Occurrences in String – Count how many times a pattern appears.
     Topic Hint: KMP/Z algorithm.

28. Word Break Problem – Check if string can be segmented into dictionary words.
     Topic Hint: DP with hash set.

29. Longest Prefix Suffix (LPS) – Longest prefix that is also a suffix.
     Topic Hint: KMP preprocessing array.

30. Find All Anagrams in String – Return indices of substrings that are anagrams of target.
     Topic Hint: Sliding window + frequency map.

31–40: Advanced String Handling

31. Convert String to Integer (atoi) – Convert string to integer with edge cases.
     Topic Hint: Parsing + boundary handling.

32. Longest Repeated Substring – Find longest substring appearing at least twice.
     Topic Hint: Suffix array/trie.

33. Shortest Palindrome – Make string palindrome by adding characters at front.
     Topic Hint: KMP trick + string reversal.

34. Word Ladder Problem – Transform one word to another with minimal steps.
     Topic Hint: BFS on dictionary graph.

35. Palindrome Partitioning – Partition string into all possible palindromic substrings.
     Topic Hint: Backtracking + DP.

36. Valid Parentheses String – Validate if parentheses string is balanced.
     Topic Hint: Stack-based matching.

37. Remove Invalid Parentheses – Remove minimum invalid parentheses.
     Topic Hint: BFS/backtracking.

38. Longest Valid Parentheses – Longest valid parentheses substring.
     Topic Hint: DP or stack.

39. Decode Ways (A=1, B=2) – Number of ways to decode numeric string.
     Topic Hint: Recursion + DP.

40. Generate All Balanced Parentheses – Generate all valid combinations of parentheses.
     Topic Hint: Backtracking.

41–50: Real-World String Applications

41. Roman to Integer – Convert Roman numeral to integer.
     Topic Hint: Mapping + traversal.

42. Integer to Roman – Convert integer to Roman numeral.
     Topic Hint: Greedy + mapping.

43. Excel Column Title → Number – Convert Excel column title to number.
     Topic Hint: Base-26 conversion.

44. Number → Excel Column Title – Reverse conversion of numbers to Excel columns.
     Topic Hint: Division + modulo math.

45. License Key Formatting – Format string into groups with dashes.
     Topic Hint: String manipulation.

46. Valid Palindrome II – Check palindrome allowing one deletion.
     Topic Hint: Two-pointer with skip condition.

47. Compare Version Numbers – Compare strings with dot-separated numbers.
     Topic Hint: Parsing + edge cases.

48. Add Binary Strings – Perform binary addition on two strings.
     Topic Hint: Simulation of addition.

49. Multiply Strings – Multiply two large number strings.
     Topic Hint: Simulation of long multiplication.

50. Valid IP Address Restoration – Restore valid IP addresses from a string.
     Topic Hint: Backtracking + parsing.

Final Thoughts

This list gives you a clear problem statement + topic hint for each of the Top 50 string problems.

Note: Use it as a prep tracker : solve, review the hint and connect each problem to its pattern.
Over time, you’ll spot string problem blueprints instantly in interviews.