The Only 21 String Problems You Need to Prepare for Coding Interviews in 2026
Master these and you’ll instantly feel more confident in your next coding interview
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Along with array, binary tree, and linked list data structures, the string is another popular topic on programming job interviews. I have never participated in a coding interview where no string-based questions were asked.
This is very obvious because I have never written a program where I have not used a single String variable. You will always find String as one of the most used data types or data structures in any program.
In this article, I am going to share some of the most common string-based coding problems I have come across in many Programming interviews I have been part of. I also have experience from both sides of the table as a candidate as well as the Interviewer, so I know how important these questions are.
Btw, there is no point in solving these questions if you don’t have basic knowledge of data structure, or you have not refreshed it in recent times. In that case, I suggest you first go through a suitable data structure and algorithm course or book to revise the concept.
This will save you a lot of time going back and forth between the book and your IDE for each question.
If you need resources, I suggest following the online courses to learn Data Structure and Algorithms. Even though they are independent of any programming language, I strongly recommend you join the class that explains problems in the programming language you are most comfortable with. Like
Data Structures and Algorithms: Deep Dive Using Java for Java developers
Algorithms and Data Structures in Python for those who love Python
JavaScript Algorithms and Data Structures Masterclass by Colt Steele for JavaScript programmers
Mastering Data Structures & Algorithms using C and C++ for those who are good at C/C++
5. Grokking the Coding Interview: Patterns for Coding Questions
Forget about the hundreds of Leetcode problems. Master these 15 underlying patterns in interview questions, and you’ll be able to tackle anything you face in an interview
How to solve String-based Coding Problems?
A good thing about the string data structure is that if you know the array data structure, you can easily solve string-based problems because strings are nothing but a character array.
So all the techniques you know by solving array-based coding questions can be used to solve string programming questions as well.
Here is my list of some of the frequently asked string coding questions from programming job interviews:
How do you reverse a given string in place? (solution)
How do you print duplicate characters from a string? (solution)
How do you check if two strings are anagrams of each other? (solution)
How do you find all the permutations of a string? (solution)
How can a given string be reversed using recursion? (solution)
How do you check if a string contains only digits? (solution)
How do you find duplicate characters in a given string? (solution)
How do you count several vowels and consonants in a given string? (solution)
How do you count the occurrence of a given character in a string? (solution)
How do you print the first non-repeated character from a string? (solution)
How do you convert a given String into an int like the atoi()? (solution)
How do you reverse words in a given sentence without using any library method? (solution)
How do you check if two strings are a rotation of each other? (solution)
How do you check if a given string is a palindrome? (solution)
How do you find the length of the longest substring without repeating characters? (solution)
Given string str, how do you find the longest palindromic substring in str? (solution)
How to convert a byte array to a string? (solution)
How to remove the duplicate character from a string? (solution)
How to find the maximum occurring character in a given String? (solution)
How do you remove a given character from a string? (solution)
Given an array of strings, find the most frequent word in a given array, I mean, the string that appears the most in the array. In the case of a tie, the string that is the smallest (lexicographically) is printed. (solution)
These questions help improve your knowledge of string as a data structure. If you can solve all these String questions without any help, then you are in good shape.
For more difficult questions, I suggest you solve problems given in the Algorithm Design Manual by Steven Skiena, a book with the toughest algorithm questions.
If you need to revise your Data Structure and Algorithms concepts, then you can also see these resources:
Data Structures and Algorithms: Deep Dive Using Java for Java developers
JavaScript Algorithms and Data Structures Masterclass by Colt Steele for JavaScript programmers
Mastering Data Structures & Algorithms using C and C++ for those who are good at C/C++
Grokking the Coding Interview: Patterns for Coding Questions
Forget about the hundreds of Leetcode problems. Master these 15 underlying patterns in interview questions, and you’ll be able to tackle anything you face in the interview. My big thanks to Arslan Ahmad and the Design Guru team for creating such an awesome course.
By the way, DesignGuru has many other Grokking courses to prepare for essential coding interview topics like OOP Design, System Design, Dynamic Programming etc and you can get access to all of their courses for a big discount by joining their All course bundle. You can also use code GURU to get 30% discount.
These are some of the best courses on data structures and algorithms, and you can choose the one that is most suitable for you. Btw, I will receive payments if you buy these courses
Now You’re Ready for the Coding Interview
These are some of the most common questions outside of data structure and algorithms that help you to do really well in your interview.
I have also shared a lot of these questions on my javarevisited and java67, so if you are really interested, you can always go there and search for them.
These common String-based questions are the ones you need to know to successfully interview with any company, big or small, for any level of programming job.
If you were looking for a programming or software development job, you could start your preparation with this list of coding questions, but you also need to prepare other topics, such as System Design, Concurrency, Object-Oriented Programming, Database Design, and Data Structures and Algorithms, like Dynamic Programming.
Thankfully, Educative has got you covered; they have detailed, in-depth courses to cover each of these topics, and you can refer to them before you go for interviews. Here are some of my recommended ones.
Grokking the System Design Interview for System Design
Grokking the Object-Oriented Design Interview for Object-Oriented Programming and Design.
Grokking Dynamic Programming Patterns for Coding Interviews to excel on dynamic programming-based coding problems.
Coderust: Hacking the Coding Interview walks you through 80+ example problems and their solutions with step-by-step visualizations — so that you are actually learning instead of blindly memorizing solutions
Java Multithreading for Senior Engineering Interviews to answer concurrency-based problems.
This list of courses provides good topics to prepare and also helps assess your preparation to find out your areas of strength and weakness. You can either buy these courses one by one or better go for the Educative subscription, which allows access to all of these courses and 100+ other software development courses.
Educative is also offering a whopping 47% discount on their yearly subscription. As a programmer, there is a lot to learn, and sometimes you want the simplicity of taking multiple courses without paying for each one. With this subscription model, you can now just pay once and get full access to every course on Educative.
Ultimately, good knowledge of these topics is essential for success in coding interviews, and that’s where you should focus most of your attention.
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