Top 20 String Algorithm Questions from Coding Interviews

In this article we are going to see top 20 String based coding interview question and their solution to help programmers better prepare for interviews. String is one of the most important data structure and available in almost every programming language e.g. Java, C, C++, Python, Perl and Ruby. Though there implement differ but essence remain same e.g. String is NULL terminated character array in C but String is object in Java, again backed by character array. String is also available on weekly typed language e.g. Python and Perl. This is why, you will always find some String based coding question on programming interview. Even in your college days, you would have solved lots of coding problems based upon String like reversing String in place, checking if String is palindrome, checking if two strings are anagram of each other, calculating permutations of String etc. Coding questions asked on Programming interview is not very different from that, but yes it gets slightly more difficult with your experience, For example, in your first few programming job interview, you may find questions like removing duplicates from String or replacing all spaces with %20, but as you get more experience and apply for senior developer position, you can expect tough questions like how to find longest palindrome in a string or printing all permutations of String etc. This article contains both easy and difficult String coding questions for your preparation. I have not posted the solution right behind the problem so that you can give it a shot before checking solution. You can solve these String based coding question on any language of your choice e.g. C, C++, Java, Python or even Perl.

20 String Algorithm based Coding Interview Questions

Here is my collection of some of the most frequently asked String based coding questions from programming interview. Remember, there are many algorithms to solve the same problem, and you should know that, mainly to handle follow-up question better. Also remember to solve same question using both recursion and iteration, as interviewer really like to ask iterative version if you come up with recursive algorithm and vice-versa. Nevertheless, if you see your favorite question is not included in list, feel free to suggest, I will include it. You can also post question asked to you on your interview and you have not find its solution yet. Some questions are still unsolved or solution is not yet posted on my blog. Also difficult level increases as you move questions.

1) How to Print duplicate characters from String? (solution)

To start with, we have a simple String related coding question frequently asked in programming interviews. You need to write a program in C, C++, Java or Python to print duplicate characters from a given String, for example if String is "Java" then program should print "a". Bonus points if your program is robust and handle different kinds of input e.g. String without duplicate, null or empty String etc. Bonus points if you also write unit tests for normal and edge cases.

2) How to check if two Strings are anagrams of each other? (solution)

A simple coding problem based upon String, but could also be asked with numbers. You need to write a Java program to check if two given strings are anagrams of Each other. Two strings are anagrams if they are written using the same exact letters, ignoring space, punctuation and capitalization. Each letter should have the same count in both strings. For example, Army and Mary are anagram of each other.

3) How to program to print first non repeated character from String? (solution)

One of the most common string interview questions: Find the first non-repeated (unique) character in a given string. for Example if given String is "Morning" then it should print "M". This question demonstrates efficient use of Hashtable. We scan the string from left to right counting the number occurrences of each character in a Hashtable. Then we perform a second pass and check the counts of every character. Whenever we hit a count of 1 we return that character, that’s the first unique letter. Be prepared for follow-up question for improving memory efficiency, solving it without hash table as well.

4) How to reverse String in Java using Iteration and Recursion? (solution)

Your task is to write a program to reverse String in Java without using StringBuffer class. You also need to provide both iterative and recursive algorithm for String reversal. You can use other String utility methods e.g. charAt(), toCharArray() or substring() from java.lang.String class.

5) How to check if a String contains only digits?  (solution)

You need to write a program to check a String contains only numbers by using Regular expression in Java. You can use Java API but a solution without using Java API will be better because that is what interviewer can always ask.

6) How to find duplicate characters in a String? (solution)

You need to write a program to print all duplicate character and their count in Java. For example if given String is "Programming" then your program should print
g : 2
r : 2
m : 2

7) How to count number of vowels and consonants in a String? (solution)

One of easiest String question you will ever see. You have to write a Java program which will take a String input and print out number of vowels and consonants on that String. For example if input is "Java" then your program should print "2 vowels and 2 consonants". If you get this question on Interview, you should clarify that whether String can contain numbers, special characters or not e.g. anything other than vowels and consonants.

8) How to count occurrence of a given character in String? (solution)

If interviewer ask you to count occurrence of more than one character than you can either use an array, hash table or any additional data structure. In order to solve this problem, you are not allowed to do so. Your method must return count of given character, for example if input String is "Java" and given character is 'a' then it should return 2. Bonus point if you handle case, null and empty String and come up with unit tests.

9) How to convert numeric String to int? (solution)

A classical coding interview question based upon String. You need to write a method like atoi() from C/C++, which takes a numeric String and return its int equivalent. For example, if you pass "67263" to the program then it should return 67263. Make sure your solution is robust i.e. it should be able to handle + and - character, null and empty String, integer overflow and other corner cases. Bonus points if you come up with good unit test cases. By the way, if your interviewer doesn't mention to you about atoi() then you can also use Java API's parseInt() or valueOf() method to solve this problem.

10) How to replace each given character to other e.g. blank with %20? (solution)

Write a Java program to replace a given character in a String to other provided character, for example if you are asked to replace each blank in a String with %20, similar to URL encoding done by browser, so that Server can read all request parameters. For example if input is "Java is Great" and asked to replace space with %20 then it should be "Java%20is%20Great".

11) How to find all permutations of String? (solution)

I have seen this String interview question on many interviews. It has a easy recursive solution but thinks get really tricky when Interviewer ask you to solve this question without using recursion. You can use Stack though. Write a program to print all permutations of a String in Java, for example if input is "xyz" then it should print "xyz", "yzx", "zxy", "xzy", "yxz", "zyx".

12) How to reverse words in a sentence without using library method? (solution)

Write a function, which takes a String word and return sentence on which words are reversed in order e.g. if input is "Java is best programming language", output should be "language programming best is Java".

13) How to check if String is Palindrome?(solution)

Another easy coding question based upon String, I am sure you must have done this numerous time. Your program should return true if String is Palindrome, otherwise false. For example, if the input is "radar", the output should be true, if input is "madam" output will be true, and if input is "Java" output should be false.

14) How to remove duplicate characters from String? (solution)

This is one of the interesting String question, which also has lots of variants. You need to remove duplicate characters from a given string keeping only the first occurrences. For example, if the input is ‘bananas’ the output will be ‘bans’. Pay attention to what output could be, because if you look closely original order of characters are retained in output. This is where many developer make mistake of shorting character array of String and removing duplicates, similar to how you remove duplicates from array. That destroys original order of characters and will not be correct solution in this case.

15) How to check if a String is valid shuffle of two String? (solution)

One more difficult String algorithm based coding question for senior developers. You are given 3 strings: first,  second, and  third.  third String is said to be a shuffle of first and second if it can be formed by interleaving the characters of first and second String in a way that maintains the left to right ordering of the characters from each string. For example, given first = "abc" and second = "def",  third = "dabecf"  is a valid shuffle since it preserves the character ordering of the two strings. So, given these 3 strings write a function that detects whether third String is a valid shuffle of first and second String.

16) Write a program to check if a String contains another String e.g. indexOf()? (solution)

You need to write a function to search for the existence of a string (target) in another string (source). The function takes two strings as the input and returns the index where the second string is found. If the target string cannot be found, then return -1. If you are a Java developer, then you can related its behavior to indexOf() method from java.lang.String class. This question is also asked as Code and algorithm to check if a given short string is a substring of a main string. Can you get a linear solution (O(n)) if possible?

17) How  to return highest occurred character in a String? (solution)

You need to write a function to implement algorithm which will accept a string of characters and should find the highest occurrence of the character and display it. For example if input is "aaaaaaaaaaaaaaaaabbbbcddddeeeeee" it should return "a".

18) Write a program to remove a given characters from String? (solution)

One of my favorite coding question, when I interview Java developers. You need to write a Java method which will accept a String and a character to be removed and return a String, which doesn't has that character e.g remove(String word, char ch).  You need to provide both iterative and recursive solution of this method and also has to write JUnit tests to cover cases like null and empty String, input which only contains letter to be removed, String which doesn't contain given character etc.

19) Write a program to find longest palindrome in a string? (solution)

This is one of the tough coding question based upon String. It's hard to think about an algorithm to solve this problem until you have practiced good. What makes it more difficult is the constraint that your solution has O(n) time complexity and O(1) space complexity.

20) How to sort String on their length in Java? (solution)

Write a Program to sort String on their length in Java? Your method should accept  an array of String and return a sorted array based upon length of String. Don't forget to write unit tests for your solution.



That's all on this list of 15 String Algorithm based coding questions. These are really good question to prepare for programming interview, not only you can expect same question on real interview but also it will prepare you how to tackle algorithmic coding interview questions. Even if you don't find same question, you would be able to apply the knowledge you gain by solving these question by yourself. Always remember, you are judged by the code you write, so always write production quality code, which would pass general test, corner cases, invalid inputs, robustness test and also pass performance test. Whenever asked to solve a coding problem, always think about all possible input and write test for that.