Strings Interview Questions & Tips for Senior Engineers

Strings Interview Questions & Tips

By Jared Skinner and Mike Mroczka | Last updated: May 27, 2025

What are Strings?

Along with arrays, strings are one of the simplest data structures out there and are always coming up. Strings can be thought of as a subset of arrays because, as simple as they are, strings come with their own set of intricacies and challenges. Here are three key properties that we need to remember as we start working with strings.

Under the hood, strings are just an array of characters. This is important because it affects the time complexities of string methods we might use – keep reading for more details on this!
Like arrays, strings are variable in size. This one might be obvious given the above point, but it bears repeating. In most languages, arrays are treated as a collection, whereas we think of a string as a single “thing”. However, remember that it’s still an array of characters in memory, so the space the string takes up AND it takes a linear amount of time to generate a hash of a string where N is proportional to the length of the string. When discussing time complexities the length of a string plays a factor.
In most languages, strings are immutable. This is the biggest difference between strings and generic arrays – you can't change them once they are constructed! If you need to update a string you have to rebuild an entirely new string with the update you want. How you avoid this waste of time & space is language dependent. In Java, a StringBuilder is used to hold a stream of characters before converting it to a final immutable string. In Python, it's common to use a list to hold individual characters and then convert it to a string at the very end. Some problems like Reversing a String are commonly solved in C++, where strings are mutable, but if you are working in a language where strings aren't mutable to follow the spirit of this question it requires you to "fake" string mutability by first converting your string to a list then pretending your language can mutate strings directly.

Common Mistakes in Interviews Featuring Strings

Viewing String Problems As "Easy"

Strings, despite being one of the first data structures introduced to programmers, shouldn't be underestimated as "easy" in interview scenarios due to their versatility. Unlike data structures like trees or graphs that have specific algorithms like DFS/BFS or backtracking respectively, strings can incorporate a range of technical topics frequently asked in interviews. Strings are a linear data structure, and because of that all common algorithms related to linear data structures could potentially be involved in a string question. This means techniques like two pointers, sliding windows, recursion, backtracking, and dynamic programming (to name just a few) can be used in a string question. Therefore, avoiding string practice due to perceived ease could lead to challenges in handling complex string problems in interviews.

Messing Up String Conversions

One of the more annoying things we can have to do, which nevertheless comes up fairly often, is various forms of string math. This includes converting characters to their index in the alphabet and converting characters to their numeric value. General familiarity with how to do these things in your interviewing language is critical. It's obscure enough to not remember on the spot, but simple enough that it looks bad if you don't know how to do it.

Making Assumptions About String Contents

It's easy to think of strings as just letters in the alphabet, but this will lead to one of the most common interview errors with string questions – making an incorrect assumption about the string contents. Useful clarifying questions to ask whenever strings are involved in a problem can be questions like.

Are we guaranteed that there will just be alphabetical characters? Alphanumeric?
Do we need to worry about punctuation?
Do I need to handle special characters/symbols?
Can the string ever be empty or null?
Is the string always lower/uppercase? Can we receive a mixed case string?
Do we need to worry about the string encoding at all? UTF-8, ASCII, etc?
Does the string fit into memory?

Here's a good example of a string question where all of these questions matter in achieving the correct output to pass all tests. Don't make assumptions about what is in the string, asking these types of questions helps demonstrate your seniority and familiarity with common gotchas.

Hidden Complexity

One danger you should be aware of is how languages will abstract away the heavy lifting when it comes to strings. This can make us wrongly assume that "simple" operations are cheap. For example, we can very easily get a substring of a string in Python using the Python slicing syntax: s[1:5]. Writing it this way we may assume that this is a constant-time operation. However, in most cases, operations like this are still going to copy the substring (remember strings are immutable), meaning that it is going to take us linear time.

Language-Specific Advice

Java Strings

Fast Facts:

Mutable? No
Primitive? No
Comparison: s1.equals(s2)
Access the ith character: s1.charAt(i)

Useful Java String Methods:

Method	Description	Time	Space
`s.length()`	Returns the length of the string	`O(1)`	`O(1)`
`s.charAt(int i)`	Returns the character at index `i`	`O(1)`	`O(1)`
`s.substring(int i, int j)`	Returns substring from `i` to `j`	`O(j-i)`	`O(j-i)`
`s.contains(String s)`	Returns `True` if `s` is contained in the string	`O(n)`	`O(1)`
`s.indexOf(String s)`	Returns the starting index of the first occurrence of `s`	`O(n)`	`O(1)`

C++ Strings

Fast Facts:

Mutable? Yes
Primitive? No
Comparison: s1.compare(s2)
Access the ith character: s1[i]

Useful C++ String Methods:

Method	Description	Time	Space
`s.length()`	Returns the length of the string	`O(1)`	`O(1)`
`s1.find(s2)`	Returns the index of `s1` in the string `s2`	`O(s1 * s2)`	`O(1)`
`s.substr(i, j)`	Get the substring of `s` from `i` with length `j`	`O(j)`	`O(j)`

Python Strings

Fast Facts:

Mutable? No
Primitive? Yes and no, but mostly no. "Primitive" isn't a word in Python, all types are objects!
Comparison: s1 == s2
Access the ith character: s1[i]

Useful Python String Methods:

Method	Description	Time	Space
`len()`	Returns the length of the string	`O(1)`	`O(1)`
`s1 in s2`	Is `s1` a substring of `s2`	`O(s1 * s2)`	`O(1)`
`s.index(s2)`	Returns the index of the first occurrence of `s2` in the string, ValueError if not found	`O(s1)`	`O(1)`

JavaScript Strings

Fast Facts:

Mutable? No
Primitive? Yes
Comparison: s1 == s2 and sometimes s1 === s2
Access the ith character: s1[i]

Useful JavaScript String Methods:

Method	Description	Time	Space
`s.length()`	Returns the length of the string	`O(1)`	`O(1)`
`s.includes(s1)`	Is `s1` a substring of `s2`	`O(s2)`	`O(1)`
`s.indexOf(searchValue)`	Returns the index of the first occurrence of `searchValue`	`O(1)`	`O(1)`

Strings Interview Questions & Tips for Senior Engineers

Strings Interview Questions & Tips

What are Strings?

Common Mistakes in Interviews Featuring Strings

Viewing String Problems As "Easy"

Messing Up String Conversions

Making Assumptions About String Contents

Hidden Complexity

Language-Specific Advice

Java Strings

C++ Strings

Python Strings

JavaScript Strings

Common String interview Questions

Longest Substring with At Most K Distinct Characters

Permutation in String

Reverse Integer

Longest Palindromic Substring

Minimum Window Substring

Simplify Path

Shuffle String

XML Parser

Longest Common Subsequence

Reverse String

Minimum Cost to Construct String

Generate Parentheses

Decode String

Prefix Pairs

Reverse Words in a String

Valid Palindrome

Infinite Binary Print

Regular Expression Matching

Longest Substring Without Repeating Characters