Top 50 Most Frequently Asked String Coding Problems (Interview Preparation )
String manipulation is one of the most important topics in coding interviews. From reversing strings to solving dynamic programming-based problems like Edit Distance, mastering string algorithms helps you stand out in technical rounds at companies like Google, Amazon, Microsoft and Meta.
This guide covers 50 of the most frequently asked string problems, categorized by difficulty level (Easy, Medium, and Hard) each with a short description, core concepts and key ideas to help you prepare smarter.
Easy Level String Problems
1. Reverse String
Description: Reverse the characters of a given string in-place.
Concepts: Two-pointer swap, in-place modification.
Example: Input: "hello" -> Output: "olleh"
2. Reverse Words in a String
Description: Reverse the order of words in a sentence without altering characters in each word.
Concepts: Splitting, trimming spaces, joining with reversed order.
3. Check Palindrome String
Description: Determine whether a string reads the same backward as forward.
Concepts: Two-pointer technique, character comparison.
4. Valid Anagram
Description: Check if two strings contain the same characters with the same frequencies.
Concepts: Sorting, frequency counting (HashMap).
Example: "anagram" vs "nagaram" -> true
5. First Unique Character in a String
Description: Find the index of the first non-repeating character.
Concepts: HashMap frequency counting, linear scan.
6. Remove All Adjacent Duplicates
Description: Continuously remove adjacent repeating characters until no duplicates remain.
Concepts: Stack-based removal approach.
7. Implement strStr()
Description: Return the index of the first occurrence of a substring (needle) in another string (haystack).
Concepts: Brute force, KMP (Knuth-Morris-Pratt) algorithm.
8. Longest Common Prefix
Description: Find the longest common prefix string among an array of strings.
Concepts: Sorting, character-by-character comparison.
9. Count Vowels and Consonants
Description: Count the number of vowels and consonants in a string.
Concepts: Iteration, set lookup.
10. Check if String is Rotation of Another
Description: Determine if one string can be obtained by rotating another.
Concepts: Concatenation trick (s1 + s1 contains s2).
11. Check if String is Subsequence of Another
Description: Check whether string s is a subsequence of string t.
Concepts: Two-pointer traversal.
12. Convert String to Integer (atoi)
Description: Parse a numeric string into an integer while handling spaces, signs, and overflow.
Concepts: Parsing logic, overflow handling.
13. Check Balanced Parentheses
Description: Verify if the parentheses/brackets in a string are balanced.
Concepts: Stack push/pop validation.
14. Find All Permutations of a String
Description: Generate all unique permutations of a given string.
Concepts: Backtracking recursion.
15. Check Isomorphic Strings
Description: Check if two strings have the same pattern of character mapping.
Concepts: Hash mapping for pattern consistency.
Medium Level String Problems
16. Group Anagrams
Description: Group together all strings that are anagrams of each other.
Concepts: Sorting character signature, HashMap grouping.
17. Longest Substring Without Repeating Characters
Description: Find the length of the longest substring without duplicate characters.
Concepts: Sliding window with character frequency map.
18. Longest Palindromic Substring
Description: Find the longest substring that is a palindrome.
Concepts: Expand around center, dynamic programming.
19. Minimum Window Substring
Description: Find the smallest substring containing all characters of another string.
Concepts: Sliding window, frequency counting.
20. Valid Parentheses String with *
Description: Check if a string with (, ) and * can be valid.
Concepts: Greedy approach, range tracking.
21. String Compression
Description: Compress repeated characters (e.g. "aaabb" -> "a3b2").
Concepts: Two-pointer traversal, in-place modification.
22. Repeated Substring Pattern
Description: Check if the given string can be formed by repeating a substring.
Concepts: Concatenation trick, string manipulation.
23. Multiply Two Strings
Description: Multiply two large numbers represented as strings.
Concepts: Simulation of digit-by-digit multiplication.
24. Longest Repeating Character Replacement
Description: Replace characters to create the longest substring with repeating characters.
Concepts: Sliding window, frequency optimization.
25. Check if Two Strings are One Edit Away
Description: Check if two strings can become equal with one insert/delete/replace.
Concepts: Two-pointer comparison.
26. Count and Say Sequence
Description: Generate the n-th term of the count-and-say sequence.
Concepts: Simulation, string building.
27. Find All Anagrams in a String
Description: Find all starting indices of anagrams of string p in string s.
Concepts: Sliding window, frequency map.
28. Check if a String is a Valid Shuffle of Two Strings
Description: Check if one string is a valid shuffle of two others.
Concepts: Sorting, interleaving validation.
29. Zigzag Conversion
Description: Convert a string into a zigzag pattern and read row by row.
Concepts: Simulation of zigzag traversal.
30. Reverse Only Letters
Description: Reverse only alphabetic characters, keeping other characters fixed.
Concepts: Two-pointer, condition-based swapping.
31. Basic Calculator (I & II)
Description: Evaluate arithmetic expressions given as strings.
Concepts: Stack-based evaluation, parsing numbers/operators.
Hard Level String Problems
32. Edit Distance (Levenshtein Distance)
Description: Find the minimum operations (insert, delete, replace) to convert one string into another.
Concepts: Dynamic programming (2D DP table).
33. Wildcard Pattern Matching
Description: Match a string with a pattern containing ? and *.
Concepts: DP table, greedy optimization.
34. Regular Expression Matching
Description: Implement regex matching with . and *.
Concepts: DP-based pattern validation.
35. Word Break Problem
Description: Check if a string can be segmented into dictionary words.
Concepts: DP with HashSet lookup.
36. Word Break II
Description: Return all possible segmentations of a string into dictionary words.
Concepts: DP + backtracking.
37. Longest Common Subsequence (LCS)
Description: Find the longest sequence present in both strings in the same order.
Concepts: DP table.
38. Longest Common Substring
Description: Find the longest continuous substring common between two strings.
Concepts: DP, suffix-based comparison.
39. Shortest Common Supersequence
Description: Find the shortest string that contains both given strings as subsequences.
Concepts: LCS-based DP approach.
40. Distinct Subsequences Count
Description: Count how many distinct subsequences of s match target t.
Concepts: Dynamic programming.
41. Minimum Insertions to Form a Palindrome
Description: Find the minimum number of insertions to make a string palindrome.
Concepts: LCS with reverse string.
42. Minimum Deletions to Make Strings Equal
Description: Find minimum deletions to make two strings equal.
Concepts: LCS-based difference calculation.
43. Palindrome Partitioning
Description: Partition a string into all possible palindromic substrings.
Concepts: Backtracking, recursion.
44. Palindrome Partitioning II (Min Cuts)
Description: Find the minimum cuts required to partition a string into palindromes.
Concepts: Dynamic programming.
45. Find All Words in a Grid (Word Search II / Boggle)
Description: Search for dictionary words in a 2D grid of letters.
Concepts: Trie data structure, DFS.
46. Smallest Window Containing All Characters of Itself
Description: Find the shortest substring containing all unique characters.
Concepts: Sliding window, character frequency.
47. String to Integer with Large Constraints
Description: Convert very large numeric strings to integers (beyond built-in limits).
Concepts: String arithmetic, modular operations.
48. KMP Algorithm (Pattern Search)
Description: Efficient substring search using a prefix-suffix table.
Concepts: Prefix table preprocessing.
49. Rabin-Karp Algorithm
Description: Search substrings using rolling hash for efficiency.
Concepts: Hashing, modular arithmetic.
50. Manacher’s Algorithm
Description: Find the longest palindromic substring in linear time O(n).
Concepts: Center expansion with radius array.
Conclusion
These 50 most commonly asked string coding problems cover everything from basic manipulation to advanced pattern matching and dynamic programming.
Mastering these will help you:
- Tackle real-world interview challenges efficiently
- Improve your algorithmic thinking
- Boost your confidence for Top Tech Companies Interview like FAANG-level interviews