Commonly Asked Data Structure Interview Questions on Strings
Last Updated :
23 Jul, 2025
Strings are essential data structures used to represent sequences of characters and are frequently encountered in coding interviews. Questions often focus on string manipulation techniques such as searching, concatenation, reversal, and substring extraction. Understanding key algorithms like pattern matching and edit distance is crucial. Mastering these concepts can help you effectively tackle string-related problems in interviews, showcasing your problem-solving abilities and algorithmic knowledge.
Theoretical Questions for Interviews on Strings
1. What is a string in programming?
A string is a sequence of characters, typically used to represent text in programming. In most languages, strings are either mutable or immutable, and they are usually stored as arrays of characters.
2. What is the difference between a character array and a string?
A character array is a collection of characters that can be modified, whereas a string is an abstract data type used to represent text, and in many languages, it may be immutable.
3. What is the time complexity of accessing a character in a string?
Accessing a character in a string is typically O(1), assuming the string is stored in a contiguous block of memory (like an array).
4. What is a substring, and how does it differ from a subsequence of a string?
A substring is a contiguous block of characters within a string, while a subsequence is a sequence derived from the string by deleting some or no characters, but the remaining characters must appear in the same order.
5. How do you reverse a string?
Reversing a string can be done by swapping the characters from both ends of the string until you reach the middle. This can be implemented using two pointers.
6. How would you find the length of a string without using built-in functions?
You can find the length of a string by iterating through each character until you encounter a null-terminating character ('\0' in C) or by counting the characters in the string.
7. What is the difference between string concatenation and string interpolation?
String concatenation is the process of joining two or more strings using operators, while string interpolation involves embedding variables or expressions within a string using special syntax.
8. What is a string buffer and how is it different from a string?
A string buffer (or StringBuilder in Java) is a mutable sequence of characters. Unlike strings, which are immutable, string buffers allow for modification without creating new objects, making them more efficient for frequent modifications.
9. What is the difference between a mutable and immutable string?
A mutable string can be modified after it's created (like in Python with list or C++'s std::string), while an immutable string cannot be changed once created (like in Java's String class or Python's str). Immutable strings provide better security and thread safety.
10. What is the purpose of the Knuth-Morris-Pratt (KMP) algorithm?
The KMP algorithm is used for pattern matching. It improves on the naive approach by avoiding unnecessary comparisons by preprocessing the pattern to create a "partial match" table (also known as the "prefix" table).
11. Explain the concept of "palindrome" and how you would check if a string is palindrome.
A palindrome is a string that reads the same backward as forward (e.g., "madam"). To check if a string is a palindrome, compare characters from the beginning and end, moving towards the center.
12. What is the time complexity of searching for a character in a string?
Searching for a character in a string is generally O(n), where n is the length of the string, because we may need to traverse the entire string in the worst case.
Top Coding Interview Questions on Strings
The following list of 50 strings coding problems covers a range of difficulty levels, from easy to hard, to help candidates prepare for interviews.
Top 50 String Coding Problems for Interviews
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