Top 40 Coding Interview Questions You Should Know

Coding Interview Questions on Conceptual Understanding

This section covers some coding interview questions that test the conceptual understanding of the candidate.

1. What is a Data Structure?

• A data structure is a storage format that defines the way data is stored, organized, and manipulated.
• Some famous data structures are Arrays, Trees, and Graphs.

2. What is an Array?

• An array is commonly referred to as a collection of items stored at contiguous memory locations.
• Items stored are of the same type.
• It organizes data so that related values can be easily sorted or searched.

Fig: Array

3. What is a Linked List?

• Like an array, a linked list refers to a linear data structure in which the elements are not necessarily stored in a contiguous manner.
• It is basically a sequence of nodes, each node points towards the next node forming a chain-like structure.

Fig: Linked List

4. What is LIFO?

• LIFO is an abbreviation for Last In First Out
• It is a way of accessing, storing and retrieving data.
• It extracts the data that was stored last first.

5. What is a Stack?

• A stack refers to a linear data structure performing operations in a LIFO (Last In First Out) order.
• In a stack, elements can only be accessed, starting from the topmost to the bottom element.

6. What is FIFO?

• FIFO stands for First In First Out.
• It is a way of accessing, storing and retrieving data.
• The data that was stored first is extracted first.

Fig: LIFO, FIFO

Till now, you’ve covered some very fundamental coding interview questions. Going ahead you will dive deeper into the subject.

7. What is a Queue?

• A queue refers to a linear data structure that performs operations in a FIFO order.
• The least recently added elements are removed first instead of a stack in a queue.

Fig: Queue

8. What are Binary Trees?

• A binary tree is an extension of the linked list structure where each node has at most two children. 
• A binary tree has two nodes at all times, a left node and a right node.

Fig: Binary Trees

9. What is Recursion?

• Recursion refers to a function calling itself based on a terminating condition.
• It uses LIFO and, therefore, uses the stack data structure.

The next couple of coding interview questions will explore your knowledge of OOPs.

10. What is the OOPs concept?

OOPs stands for Object-Oriented Programming System, a paradigm that provides concepts such as objects, classes, and inheritance.

11. What are the concepts introduced in OOPs?

The following are the concepts introduced in OOPs:

• Object - A real-world entity having a particular state and behavior. We can define it as an instance of a class.
• Class - A logical entity that defines the blueprint from which an object can be created or instantiated.
• Inheritance - A concept that refers to an object gaining all the properties and behaviors of a parent object. It provides code reusability.
• Polymorphism - A concept that allows a task to be performed in different ways. In Java, we use method overloading and method overriding to achieve polymorphism.
• Abstraction - A concept that hides the internal details of an application and only shows the functionality. In Java, we use abstract classes and interfaces to achieve abstraction.
• Encapsulation - A concept that refers to the wrapping of code and data together into a single unit.

This is a widespread coding interview question that often allows the interviewer to explore related topics based on the candidate’s answers.

12. Explain what a Binary Search Tree is.

• A binary search tree stores data and retrieves it very efficiently. 
• The left sub-tree contains nodes whose keys are less than the node’s key value.
• The right sub-tree contains nodes whose keys are greater than or equal to the node’s key value.

Fig: Binary Search Tree

13. Explain Doubly Linked Lists?

• Doubly linked lists are categorized as a particular type of linked list in which traversal across the data elements can be done in both directions. 
• This is made possible by the presence of two links in every node, one that links to the node next to it and another that connects to the node before it.

Fig: Doubly Linked List

14. What is a Graph?

• A graph is a particular type of data structure that contains a set of ordered pairs.
• The ordered pairs in a graph are also known as edges or arcs. They are most commonly used to connect nodes where data can be stored and retrieved.

15. Differentiate between linear and non-linear data structure?

16. What is a Deque?

• A deque is a double-ended queue.
• This is a structure in which elements can be inserted or removed from either end.

17. What’s the difference between Stack and Array?

18. Which sorting algorithm is the best?

• There are many types of sorting algorithms: bubble sort, quick sort, balloon sort, merge sort, radix sort, and more.
• No algorithm can be considered the best or fastest because they have been designed for a specific type of data structure where it performs the best.

19. How does variable declaration affect memory?

• The amount of memory to be reserved or allocated depends on the data type stored in that variable. 
• For example, if a variable is declared to be “integer type”, 32 bits of memory storage will then be reserved for that particular variable.

20. What are dynamic data structures?

Dynamic data structures have the feature where they expand and contract as a program runs. It provides a very flexible data manipulation method because it adjusts based on the data size to be manipulated.

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Programming Interview Questions

The next set of coding interview questions focus tests the programming expertise of the candidates and dives deep into various related aspects.

The code screenshots given along with the below coding interview questions helps you provide the answer to the question, with clarity.

21. How do you reverse a string in Java?

• Declare a string.
• Take out the length of that string.
• Loop through the characters of the string.
• Add the characters in reverse order in the new string.

String str = "hello";
String reverse = "";
int length = str.length();
for (int i = 0; i < length; i++) {
     reverse = str.charAt(i) + reverse;
}
System.out.println(reverse);

22. How do you determine if a string is a palindrome?

• A string is a palindrome when it stays the same on reversing the order of characters in that string.
• It can be achieved by reversing the original string first and then checking if the reversed string is equal to the original string.

if (str.equals(reverse)) {
    System.out.println("Palindrome");
} else {
    System.out.println("Not Palindrome");
}

23. Find the number of occurrences of a character in a String?

To find the number of occurrences, loop through the string and search for that character at every iteration; whenever it is found, it will update the count.

int count = 0;
char search = 'a';
for (int i = 0; i < length; i++) {
    if (str.charAt(i) == search) {
        count++;
    }
}
System.out.println(count);

24. How to find out if the given two strings are anagrams or not?

Two strings are anagrams if they contain similar characters in a varied sequence.

• Declare a boolean variable that tells at the end of the two strings are anagrams or not.
• First, check if the length of both strings is the same, if not, they cannot be anagrams.
• Convert both the strings to character arrays and then sort them.
• Check if the sorted arrays are equal. If they are equal, print anagrams, otherwise not anagrams. 

boolean anagrmstat = false;
if (str.length() != reverse.length()) {
    System.out.println(str + " and " + reverse + " not anagrams string");
} else {
    char[] anagram1 = str.toCharArray();
    char[] anagram2 = reverse.toCharArray();
    Arrays.sort(anagram1);
    Arrays.sort(anagram2);
    anagrmstat = Arrays.equals(anagram1, anagram2);
}
if (anagrmstat == true) {
    System.out.println(" anagrams string");
} else {
    System.out.println(" not anagrams string");
}

25. How do you calculate the number of vowels and consonants in a String?

• Loop through the string.
• Increase the vowel variable by one whenever the character is found to be a vowel, using the if condition. Otherwise, increment the consonant variable.
• Print the values of both the vowel and the consonant count.

int vowels = 0;
int consonants = 0;
for (int k = 0; k < str.length(); k++) {
    char c = str.charAt(k);
    if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u')
        vowels++;
    else
        consonants++;
}
System.out.println("Vowel count is " + vowels);
System.out.println("Consonant count is: " + consonants);

26. How do you get the matching elements in an integer array?

• Declare an array.
• Nest a couple of loops to compare the numbers with other numbers in the array.
• Print the matching elements if found.

int[] a = { 1, 2, 3, 4, 5, 1, 2, 6, 7 };
for (int m = 0; m < a.length; m++) {
    for (int n = m + 1; n < a.length; n++) {
        if (a[m] == a[n])
            System.out.print(a[m]);
    }
}

27. How would you implement the bubble sort algorithm?

• Declare an array.
• Nest a couple of loops to compare the numbers in the array.
• The array will be sorted in ascending order by replacing the elements if found in any other order.

int[] a = { 1, 2, 7, 6, 4, 9, 12 };
for (int k = 0; k < a.length; k++) {
    for (int l = 0; l < a.length - l - 1; l++) {
        if (a[l] > a[l + 1]) {
            int t = a[l];
            a[l] = a[l + 1];
            a[l + 1] = t;
        }
    }
}

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28. How would you implement the insertion sort algorithm?

• We assume the first element in the array to be sorted. The second element is stored separately in the key. This sorts the first two elements. You can then compare the third element with the ones on its left. This process will continue until we sort the array.

int[] a = { 1, 2, 7, 6, 4, 9, 12 };
for (int m = 1; m < a.length; m++) {
    int n = m;
    while (n > 0 && a[n - 1] > a[n]) {
        int k = a[n];
        a[n] = a[n - 1];
        a[n - 1] = k;
        n--;
    }
}

29. How do you reverse an array?

• Loop till the half-length of the array.
• Replace the numbers corresponding to the indexes from the start and the end.

int[] a = { 1, 2, 7, 6, 4, 9, 12 };
for (int t = 0; t < a.length / 2; t++) { 
    int tmp = a[t]; 
    a[t] = a[a.length - t - 1]; 
    a[a.length - t- 1] = tmp; 
}

30. How would you swap two numbers without using a third variable?

• Declare two variables and initialize them with values.
• Make b the sum of both numbers.
• Then subtract the sum (b) from a, so a is now swapped.
• Lastly, subtract a from the sum (b), so b is also swapped.

int a = 10;

int b = 20;

b = b + a; // now b is sum of both the numbers

a = b - a; // b - a = (b + a) - a = b (a is swapped)

b = b - a; // (b + a) - b = a (b is swapped)

31. Print a Fibonacci series using recursion?

• The Fibonacci numbers are the numbers in the following integer sequence:

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ……..

• We can calculate them using the mathematical formula used in the Fibonacci recursive function.

public static int fibonacci(int n) 
{
    if (n <= 1)
        return n;
    return fibonacci(n - 1) + fibonacci(n - 2);
}
public static void main(String args[]) {
    int n = 10;
    System.out.println(fibonacci(n));
}

32. How do you find the factorial of an integer?

• A factorial is a function that multiplies a number by every number below it. For example, 5!= 5*4*3*2*1=120.
• Recursive function multiples the numbers until it reaches 1.

public static long factorial(long n) 
{
if (n == 1)
    return 1;
else
    return (n * factorial(n - 1));
}

33. How do you reverse a Linked List?

• Declare a linked list.
• Add elements to that linked list.
• Apply the descending iterator method to the linked list.
• This reverses the order of elements in the linked list.

LinkedList<Integer> ll = new LinkedList<>();

ll.add(1);

ll.add(2);

ll.add(3);

System.out.println(ll);

LinkedList<Integer> ll1 = new LinkedList<>();

ll.descendingIterator().forEachRemaining(ll1::add);

System.out.println(ll1);

34. How would you implement Binary Search?

• Binary search divides the array into half in every iteration step until it finds the element.
• It works on the sorted arrays since it compares the values of adjacent elements and then calculates the mid number.
• If the value of low becomes greater than high at any point, it means the element is not present in the list.

int mid = (low + high) / 2;
while (low <= high) {
    if (arr[mid] < key) {
        low = mid + 1;
    } else if (arr[mid] == key) {
        return mid;
    } else {
        high = mid - 1;
    }
    mid = (low + high) / 2;
}
if (low > high) {
    return -1;
}
return -1;

35. How would you find the second largest number in an array?

• Loop through the array.
• If the value of i is greater than the highest, store the value of i in highest, and store the value of highest in the second-highest variable.

private static int findSecondHighest(int[] array) {
    int highest = Integer.MIN_VALUE;
    int secondHighest = Integer.MIN_VALUE;
    for (int i : array) {
        if (i > highest) {
            secondHighest = highest;
            highest = i;
        } else if (i > secondHighest) {
            secondHighest = i;
        }
    }
    return secondHighest;
}

36. How do you remove all occurrences of a given character from the input string?

• Use the built-in string method “replace” to replace a character with any other character, including symbols and white spaces.

String str1 = "Australia";

str1 = str1.replace("a", "");

System.out.println(str1); // ustrli

37. Showcase Inheritance with the help of a program?

• The class Cat inherits the property color from the class Animal by extending the parent class (Animal).
• This way a class Cat can have more parent classes if it wishes to inherit their properties.

class Animal {
    String color;
}
class Cat extends Animal {
    void meow() {
        System.out.println("Meow");
    }
}

38. Explain overloading and overriding with the help of a program?

Overloading:

When a class has two or more methods with the same name, they are called overloaded methods.

class Foo {
    void print(String s) {
        System.out.println(s);
    }
    void print(String s, int count) {
        while (count > 0) {
            System.out.println(s);
            count--;
        }
    }
}

Overriding:

When a superclass method is also implemented in the child class, it’s a case of overriding.

class Base {
    void printName() {
        System.out.println("Base Class");
    }
}
class Child extends Base {
    @Override
    void printName() {
        System.out.println("Child Class");
    }
}

39. How do you check if the given number is prime?

• Use if statements to check for each condition separately:
• If the number is 0 or 1, it cannot be prime.
• If the number is 2, it is prime number.
• If the number is indivisible by other numbers, it is prime.

public static boolean isPrime(int n) {
    if (n == 0 || n == 1) {
        return false;
    }
    if (n == 2) {
        return true;
    }
    for (int i = 2; i <= n / 2; i++) {
        if (n % i == 0) {
            return false;
        }
    }
    return true;
}

40. How do you sum all the elements in an array?

• Use for loop to iterate through the array and keep adding the elements in that array.

int[] array = { 1, 2, 3, 4, 5 };
int sum = 0;
for (int i : array)
    sum += i;
System.out.println(sum);

As you prepare for your job interview, we hope these Coding Interview Questions have provided more insight into the types of questions you will likely encounter.

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If you are looking for more information, check out our YouTube video. It further explains these coding interview questions and answers and helps you become more confident while appearing for the interview.