In the ever-competitive world of tech interviews, standing out from the crowd is a true challenge. However, one resource that has gained immense popularity among aspiring programmers is Arden Dertat’s collection of interview questions. With a focus on problem-solving and algorithmic thinking, these 21 questions have become a proving ground for many candidates. In this comprehensive guide, we’ll delve into each question, providing expert insights and strategies to help you excel during your next interview.

## 1. Array Pair Sum

The premise of this question is straightforward: given an integer array and a target sum, find all pairs of elements that add up to that sum. This problem tests your ability to work with arrays and implement efficient algorithms. One possible solution is to use a hash table (or a dictionary in Python) to store the complements of each element, allowing for constant-time lookups.

## 2. Matrix Region Sum

This question introduces a twist to the classic “sum of matrix” problem. You’re given a matrix of integers and coordinates of a rectangular region within the matrix. The challenge is to find the sum of numbers falling inside the specified rectangle, and your program may be called multiple times with different regions from the same matrix. This tests your ability to optimize for repeated operations and consider space-time trade-offs.

## 3. Largest Continuous Sum

Given an array of positive and negative integers, this question asks you to find the largest continuous sum – the maximum sum of a contiguous subarray within the given array. This classic problem can be solved using dynamic programming or Kadane’s algorithm, which efficiently identifies the maximum subarray sum in linear time.

## 4. Find Missing Element

In this problem, you’re given two arrays: the first is an array of non-negative integers, and the second is formed by shuffling the elements of the first array and deleting a random element. Your task is to find the missing element in the second array. This question tests your understanding of set operations and your ability to identify patterns or leverage properties of the given arrays.

## 5. Linked List Remove Nodes

This question involves working with linked lists, a fundamental data structure in computer science. Given a linked list of integers and a target value, you must delete every node containing that value. This problem assesses your ability to traverse and manipulate linked lists while maintaining proper memory management.

## 6. Combine Two Strings

In this problem, you’re given three strings: str1, str2, and str3. The challenge is to determine whether str3 is a valid shuffle of str1 and str2. A shuffle is formed by interleaving the characters of the two strings while maintaining their relative order. This question tests your understanding of string manipulation and your ability to implement efficient algorithms.

## 7. Binary Search Tree Check

Given a binary tree, this question asks you to check whether it’s a valid binary search tree (BST) or not. This problem assesses your understanding of tree data structures and your ability to traverse and validate the properties of a BST.

## 8. Transform Word

This question introduces a more complex problem: given a source word, target word, and an English dictionary, you must transform the source word into the target word by changing, adding, or removing one character at a time, while ensuring that all intermediate words are valid English words. The goal is to find the transformation chain with the smallest number of intermediate words. This problem tests your ability to work with graphs, implement breadth-first or depth-first search, and optimize your solution.

## 9. Convert Array

This question tests your ability to manipulate arrays in-place using constant extra space. Given an array of the form [a1, a2, …, aN, b1, b2, …, bN, c1, c2, …, cN], you must convert it to [a1, b1, c1, a2, b2, c2, …, aN, bN, cN]. This problem requires a deep understanding of array indexing and in-place manipulation techniques.

## 10. Kth Largest Element in Array

Given an array of integers, this question asks you to find the kth largest element in the sorted order (not the kth distinct element). This problem tests your ability to implement efficient sorting algorithms or use data structures like heaps or quickselect to find the kth largest element without explicitly sorting the entire array.

## 11. All Permutations of String

This classic problem requires you to generate all permutations of a given string. It tests your understanding of backtracking algorithms and your ability to generate and manage recursive solutions efficiently.

## 12. Reverse Words in a String

Given an input string, this question asks you to reverse all the words while preserving their relative order. This problem tests your ability to manipulate strings and implement efficient algorithms for string reversal and word separation.

## 13. Median of Integer Stream

In this problem, you’re given a stream of unsorted integers, and you must find the median element in sorted order at any given time. This means that you don’t know the stream length in advance, and you’ll be asked to find the median multiple times. This question tests your ability to work with data streams, implement efficient sorting algorithms, and manage memory efficiently.

## 14. Check Balanced Parentheses

Given a string of opening and closing parentheses (round brackets, square brackets, and curly brackets), this question asks you to check whether the parentheses are balanced. This problem tests your ability to work with stacks and implement efficient algorithms for parenthesis matching.

## 15. First Non-Repeated Character in String

This question asks you to find the first non-repeated (unique) character in a given string. This problem tests your ability to work with hash tables or other data structures to keep track of character frequencies and your understanding of string manipulation algorithms.

## 16. Anagram Strings

In this problem, you’re given two strings, and you must determine whether they’re anagrams of each other or not. Two strings are anagrams if they’re written using the same exact letters, ignoring spaces, punctuation, and capitalization. This question tests your understanding of string manipulation, character frequency tracking, and your ability to implement efficient algorithms for anagram detection.

## 17. Search Unknown Length Array

Given a sorted array of unknown length and a number to search for, this question asks you to return the index of the number in the array. However, there’s a catch: accessing an element out of bounds will throw an exception. If the number occurs multiple times, you must return the index of any occurrence. If it isn’t present, you must return -1. This problem tests your ability to work with arrays of unknown length and implement efficient search algorithms while handling edge cases.

## 18. Find Even Occurring Element

In this problem, you’re given an integer array where one element occurs an even number of times, and all others have odd occurrences. Your task is to find the element with even occurrences. This problem tests your understanding of bit manipulation techniques, XOR operations, and your ability to identify patterns in data.

## 19. Find Next Palindrome Number

Given a number, this question asks you to find the next smallest palindrome larger than the given number. For example, if the number is 125, the next smallest palindrome is 131. This problem tests your understanding of number theory, palindrome properties, and your ability to implement efficient algorithms for number manipulation.

## 20. Tree Level Order Print

Given a binary tree of integers, this question asks you to print the tree in level order, with spaces between the numbers in the same level and new lines between different levels. This problem tests your understanding of tree traversal algorithms and your ability to implement breadth-first search or level-order traversal.

## 21. Tree Reverse Level Order Print

Similar to the previous question, this problem asks you to print the binary tree in level order, but starting from the bottom level to the root. This problem tests your understanding of tree traversal algorithms and your ability to implement breadth-first search or level-order traversal with a twist.

By thoroughly understanding and practicing these 21 interview questions, you’ll be well-equipped to tackle a wide range of algorithmic and problem-solving challenges during your interviews. Remember, the key to success lies in a deep understanding of data structures and algorithms, coupled with the ability to implement efficient and optimized solutions. With dedication and perseverance, you’ll be able to master these questions and showcase your programming prowess to potential employers.