Amazon, Wipro, Infosys and other tech organisations usually stress on the string coding interview questions for their written and telephonic interviews.

So what is a string? Well, a string problem basically refers to rearranging texts, and alphabets in an order so that the given sequence of letters makes sense. 

Questions for the coding interviews are mainly focused from the string section of data structure and algorithms.

From solving the DSA sheets to practicing via "Cracking the coding interview", approaching string interview questions has been made a lot easier and fun.

So what are you waiting for? Grab a notebook and start solving these sets of questions from string interviews.

 For your convenience, these sections have been arranged based on their level of difficulty.

What is a string problem?

One of the first things we learn in coding is how to make a string. A string is a series of characters that are strung together to form a text message, sentence, or paragraph. 

You can use a string to store information like names, numbers, or words. You can also use a string to represent an action like typing in a password or opening a window.

Now arriving at the main purpose of the blog that you are here for, string interview problems can be attempted and solved using different methods and approaches that we will shortly discuss.

In the following section we have tried to attempt the string questions at three different levels i.e Easy, Moderate and Difficult.

Level 1: Easy string interview questions

It might be tempting to read every possible question and memorize the answers, but that's not realistic. Interviewers are always trying to find new questions or questions that are not available online. Instead, you should use these questions to practice basic string concepts.

As you go through each question, try to recreate the situation you'll encounter in the interview. Start creating your own solution within a set period of time without external resources.

Easy level string questions

Problem Statement:

You have been provided a string, the challenge is to remove all the vowels from the given string and print the sequence such that it does not constitute any vowels.

Attempting the problem:

A loop is designed that iterates over the list consisting of the characters in that string, removes the vowels, and then concatenates them.

Time complexity:

Here the time complexity is the whole length of the string.

Problem Statement:

Given a valid (IPv4) Internet Protocol address S, the task is to find a fortified version of the IP address.

IP address: Each dot "." is replaced with "[.]".

Attempting the problem:

Iterate through the string and return each character within the string to the final response string, except if the present character is ".", then add "[.]" to the final response string.

Problem Statement:

You have been given two things, the string "S" and the array "Indices". Array index contains index. Then we need to shuffle the string s such that the character at the i-th index of the string s is moved to the [i]-th position of the shuffled string.

Attempting the problem:

Analyzing this problem shows that each character in s can be moved to a new position in the output string, which is not a move (like s[i] moves to index j and s[j] moves ) for a string of n characters, the minimum operation required is O(n) , because every character must be touched.

Level 2: Moderate level problems

Here are some intermediate-level questions that are commonly asked in video calls and face-to-face interviews. When prompted, you should be ready to write code or sketch the solution on a whiteboard.

Moderate level string questions

Problem Statement:

You have been given a string n, figure out the length of the longest substring within the sequence without repeating the characters of the string.

Attempting the problem:

We will be employing the sliding window approach to solve this problem. Since we're using the Sliding Window pattern, we need to keep track of window start (windowStart) and window end (windowEnd). 

The reason we are using a "sliding window" is because we are trying to get a contiguous string, aka substrings, not substrings.

We also need to be able to keep track of the indices of individual characters in the hashmap {} and recognize the possible windowStart to find the maxLength and avoid possible repeated characters.

Problem Statement:

Given a string of digits from 2 to 9, returns all possible character combinations that the digits can represent. Returns the answers in any order.

The mapping from numbers to letters (similar to phone buttons) is shown below. Note that 1 doesn't map to any character.

Attempting the problem:

Since each digit can potentially mean one of several characters, we need to write code that branches off various paths as it traverses the sequence of digits (D) entered.

Obviously this requires a depth-first search (DFS) approach, as each permutation of the characters is examined and stored in the answer array (ans). For the DFS approach, one of several options can be used, but the recursive solution is usually the cleanest.

Level 3: Difficult level string question

As with the middle section, this more difficult question can be asked in person or in a video call interview, and you'll probably get more time if you're asked to come up with a full solution.

Difficult level string question

Problem Statement:

Given an input string s and a pattern p , implement regular expression matching with support for . and where:

• Matches any character.
• Matches 0 or more of the previous item.
• Matches must cover the entire input string (not part of it).

Attempting the problem:

Let's look at some examples to understand the approach -

s = "aa" and p = "aa" Matches because all characters in s and p are the same.

Problem Statement:

Given a string S and a string T, find the smallest window in S containing all characters in T of complexity O(n).

Attempting the problem:

Since the problem requires finding a minimum window, the first paradigm that comes to mind for solving this problem is a sliding window.

Wrapping Up

To crack coding interviews for high tech companies such as Wipro, Infosys or Accenture, the DSA sheet is your holy grail.

This sheet consists of concepts and approaches related to data structure and algorithms which can help prepare for your coding interview.