string manipulation interview questions

A string is generally understood as a data type and is often implemented as an array of bytes (or words) that stores a sequence of elements, typically characters. String datatypes have historically allocated one byte per character. In C, the length of a string can be stored implicitly by using a special terminating character; often this is the null character (NUL), which has all bits zero.

How to solve String-based Coding Problems
  • 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)

The 5 String Interview Patterns You Need to Know

4.1 What is a string?

A string is an ordered sequence, or string, of characters. It is usually considered a data type and is often included as part of language primitives. In most languages, strings are implemented using an array of bytes. The bytes are encoded using some character encoding. Earlier systems used ASCII encoding, with Unicode encoding used in later systems.

C++ provides a mutable string, meaning the string contents can be changed after creation.

Python and Java provide an immutable string type. Immutable strings require a new string to be created if any changes are made. This has performance implications for string concatenation when joining a number of strings, running in quadratic time.

Java provides a mutable StringBuilder class, which should be used for concatenating multiple strings. In Python, generally the `.join` method on strings should be used, which accepts an iterable of multiple strings. Using Javas StringBuilder or Python’s `.join` provides a linear time solution for multiple string concatenations.

Another class of strings are C-style strings, so named as they are used in the C language. These strings are simple arrays. A special terminating character is stored directly after the last character of the string. This terminating character marks the end of the string within the array, or buffer. C++ has support for C-style strings, but it is preferable to use C++ style strings in pure C++ code.

Java, Python and C++ strings are more complex data structures, with class or struct methods for manipulating and querying them.

There are generally two types of string implementations: null-terminated strings (C strings), and non-null-terminated strings. The principal difference between these types of strings is how the termination and tracking the length of the string is handled.

Null-terminated strings (C strings) are character arrays terminated with a null (NUL) character, typically a byte with all bits set to 0. The length of the string is calculated by counting the characters in the string up to the terminating character. Care must be taken in working with C-style strings, as writing and reading past the end of the string terminator can result in buffer overflow errors, causing unexpected behaviour and security issues by overwriting or reading unrelated memory contents.

Non-null-terminated strings, as used in C++, Java and Python implementations, have the underlying array and the length stored separately. In Java this is a class with private fields for the character array and length, which are not directly accessible from outside the class.

C++, Java and Python strings have useful class and instance methods to work with strings. Commonly used are:

  • Substring queries to return a new string from a subset of the original string
  • Replace methods to return a new string with a specific sequence of character substituted for another
  • String formatters to create strings from a template and data variables
  • Trimming methods to create new strings without leading or trailing whitespace
  • Upper and lower case conversion methods
  • Methods to check if a string contains a given substring
  • For more advanced pattern matching in strings, Regular Expressions, or regexes, are commonly used. Most modern string implementations have support for regex queries.

    A trie, or prefix tree, is a type of search tree, often used with strings. The strangely spelt name is from “reTrieval,” but is mostly pronounced as “try.” Tries are useful for answering questions like, “Given an input sequence cod, what possible words could be formed?” This makes tries useful for auto-completing words in text input in a range of use cases, including search and texting.

    A trie data structure is made of nodes indexed with a single character key. Each node has references to its child nodes. The number of child nodes is bounded by the number of unique characters in the alphabet space making up the stored words. Often this is described as 26 for the English language, but it can be larger when taking into consideration accented characters, or if there are numeric and special characters in the alphabet space. Each node also has a flag signaling the end of a word, represented by a blue node below:

    string manipulation interview questions

    Since each node only contains one character, the complete key to that node is distributed from its parents down to that node. Retrieval of a key is a depth-first search of the trie. A search for a key ends when the end of the word node is reached. Inserting a new key is done by following the trie through each character of the new key, and adding each character as a trie node where needed, and marking the node of the last character as the end of the word.

    Inserting and searching is O(keySize) time. Space requirements are much larger than a simple string though, at O(AlphabetSpace*keySize*n), n being the total number of keys in the trie.

    1. Easy string interview questions

    You might be tempted to try to read all of the possible questions and memorize the solutions, but this is not feasible. Interviewers will always try to find new questions, or ones that are not available online. Instead, you should use these questions to practice the fundamental concepts of strings.

    As you consider each question, try to replicate the conditions you’ll encounter in your interview. Begin by writing your own solution without external resources in a fixed amount of time.

    If you get stuck, go ahead and look at the solutions, but then try the next one alone again. Don’t get stuck in a loop of reading as many solutions as possible! We’ve analyzed dozens of questions and selected ones that are commonly asked and have clear and high quality answers.

    Here are some of the easiest questions you might get asked in a coding interview. These questions are often asked during the ‘phone screen’ stage, so you should be comfortable answering them without being able to write code or use a whiteboard.

    2. Medium string interview questions

    Here are some moderate-level questions that are often asked in a video call or onsite interview. You should be prepared to write code or sketch out the solutions on a whiteboard if asked.

    FAQ

    How do you solve a string coding question?

    We will definitely try to solve your problem.
    • How to check if String is Palindrome?
    • How to find all permutations of String?
    • A Program to check if strings are rotations of each other or not?
    • How to count a number of vowels and consonants in a String?
    • How to count the occurrence of a given character in String?

    What is string in C language interview questions?

    Java String Example
    1. public class StringExample{
    2. public static void main(String args[]){
    3. String s1=”java”;//creating string by Java string literal.
    4. char ch[]={‘s’,’t’,’r’,’i’,’n’,’g’,’s’};
    5. String s2=new String(ch);//converting char array to string.
    6. String s3=new String(“example”);//creating Java string by new keyword.

    How do you solve a string question in Java?

    Java String Example
    1. public class StringExample{
    2. public static void main(String args[]){
    3. String s1=”java”;//creating string by Java string literal.
    4. char ch[]={‘s’,’t’,’r’,’i’,’n’,’g’,’s’};
    5. String s2=new String(ch);//converting char array to string.
    6. String s3=new String(“example”);//creating Java string by new keyword.

    How do you manipulate a string in Java?

    Java String Example
    1. public class StringExample{
    2. public static void main(String args[]){
    3. String s1=”java”;//creating string by Java string literal.
    4. char ch[]={‘s’,’t’,’r’,’i’,’n’,’g’,’s’};
    5. String s2=new String(ch);//converting char array to string.
    6. String s3=new String(“example”);//creating Java string by new keyword.

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