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100+ Java Algorithms Interview Questions for Freshers and Experienced Developers

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Practicing Java algorithms interview questions is essential for any Java interview preparation. The data structure and algorithm interview questions can be solved in multiple languages but sometimes involve questions that target a specific language. This article will go through some Java algorithm interview questions to assist you in preparing for your upcoming coding interviews. 

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In this article, we’ll learn:

  • Java Data Structures and Algorithms Topics
  • Java Algorithms Interview Questions and Answers
  • Java Algorithms Interview Questions for Beginners
  • Java Algorithms Interview Questions for Experienced Developers
  • FAQs on Java Algorithms Interview Questions

Java Data Structures and Algorithms Topics

While thorough preparation is essential, ensuring that you are well-versed in some key topics is a useful checklist to have. The important data structures and algorithm topics are:

  • Strings
  • Arrays
  • Sorting algorithms
  • Searching algorithms
  • Bit Manipulation
  • Graphs
  • Dynamic Programming
  • Linked List
  • Tree, Binary Tree, and Binary Search Tree
  • Number Theory

You can check out more relevant articles like Stability in Sorting Algorithms and Time and Space Complexities of Sorting Algorithms Explained on Interview Kickstart’s learn page.

Java Algorithms Interview Questions and Answers

Let’s take a look at some popular Java algorithms interview questions that you can practice for your tech interview:

Q1. What are the different types of tree traversal algorithms?

There are broadly four types of tree traversals, and each type has algorithms that help us achieve that type of traversal. The four types are inorder traversal, preorder traversal, postorder traversal, and level order traversal.

Q2. What are some of the most used sorting algorithms?

This is a commonly asked Java algorithms interview question. It tests your basic conceptual knowledge. Here’s how you can answer this: Quicksort, merge sort, heap sort, and insertion sort are some of the most used sorting algorithms. 

Q3. What are some examples of greedy algorithms?

Travelling Salesman problem, Knapsack problem, job scheduling problem, Prim's, Kruskal's, and Dijkstra's Minimum Spanning Tree algorithms, Map coloring in Graphs, and Vertex cover in graphs are some examples that utilize greedy algorithms.

Q4. Give some examples of divide and conquer algorithms.

Binary Search, Merge Sort, Quick Sort, and Strassen's Matrix Multiplication are some examples of divide and conquer algorithms.

Q5. What is the fastest searching algorithm?

Binary search is usually the fastest searching algorithm and is most widely used for efficient searching as well.

Java Algorithms Interview Questions for Beginners 

We’ll now look at some basic Java algorithm coding interview questions for freshers that you should definitely practice to ramp up your preparation. Let us jump right in!

Q1. How would you: 

  • Count leaf nodes of a binary tree
  • Print leaf nodes of a binary tree
  • traverse a binary tree
  • Check if array elements are consecutive
  • Rotate an array by P positions
  • Write an algorithm to buy and sell stocks to maximize profit
  • Count zeros in a sorted Binary Array
  • Search for a given element in a row- and column-wise sorted MxN matrix
  • Count the frequency of each element in a given sorted array
  • Separate the odd and even elements in an array
  • Swap two integers A and B without swapping the temporary variable
  • Find all the possible words in a board of characters 
  • Insert a node in a link list
  • Delete a node in a link list
  • Insert a node in an already sorted linked list in Java
  • Merge a link list into another linked list at an alternate position
  • Search an element in a rotated and sorted array

Q2. How would you find:

  • All subsets of set or power set in Java
  • The Longest Common Prefix or LCP, in Java in an array of strings 
  • The Contiguous Subarray with Sum equal to a target value in an array
  • The largest sum in a contiguous subarray
  • The largest difference between two elements in an array where the larger element comes after the smaller number.
  • The first element that repeats in an array of integers
  • The largest element in an array 
  • The smallest element in an array 
  • The smallest element in an already sorted and rotated array
  • All the subarrays with a target sum in an array
  • Permutations of an Array in Java
  • A missing number in an array
  • All the pairs of elements from an array which sum to a given number 
  • The second-largest number in an array
  • The number occurring even number of times in an array
  • A pair whose sum is closest to zero in an array

Q3. Given a sorted array and a number N, find the pair in the array whose sum is closest to N.

Q4. Given an array containing zeroes, ones, and twos only, sort the array with an algorithm that gives O(n) time complexity.

Java Algorithms Interview Questions for Experienced Developers

Here are some more frequently asked Java algorithms interview questions for experienced developers to help you be sure of your interview preparation:

Q1. How would you find:

  • The middle element of a linked list in one pass
  • If a linked list has a loop
  • The third element from the end in a linked list in one pass
  • The one duplicate number in an array that is supposed to have numbers from 1 to 100
  • If a number is a palindrome
  • The duplicate characters in a string
  • The first non-repeated character in a string
  • All the substrings of a string in Java
  • Length of a String without using any inbuilt method in java
  • The level of a node in a binary tree
  • The lowest common ancestor in a binary tree
  • An element in a sorted array using binary search

Q2. How would you:

  • Reverse a linked list using recursion and iteration
  • Implement a stack in Java
  • Reverse a string
  • Sort a stack using another stack
  • Implement a queue using a stack in Java
  • Check if a linked list is a palindrome or not
  • Implement a doubly linked list in Java
  • Reverse a linked list in pairs
  • Count subtrees with Sum equal to the target in a binary tree
  • Check if a given binary tree is a binary search tree
  • Implement trie data structure in java

Q3. Write a program to check if a string:

  • Is an anagram
  • Is a palindrome
  • Has all distinct characters
  • Is rotation of another string in Java

Q4. Implement a stack using:

  • Array
  • Linked list
  • Two queues

Q5. In a linked list, how would you find:

  • The middle element
  • Nth element from the beginning
  • Nth element from the end
  • If it has a loop
  • The starting element of a loop, if the linked list has a loop
  • The intersection of two linked lists

Q6. For a binary tree, write an algorithm to do:

  • Level order traversal
  • Spiral order traversal
  • Boundary traversal

Q7. For a binary search tree, how would you:

  • Insert a node
  • Delete a node
  • Find min and max elements in it
  • Find the lowest common ancestor
  • Find the inorder successor

Q8. How would you convert the following into a balanced binary search tree:

  • A sorted array
  • A sorted linked list

Q9. Implement the following sorting algorithms in Java:

  • Bubble sort
  • Insertion sort
  • Merge sort
  • Bucket sort
  • Radix sort
  • Selection sort
  • Heapsort
  • Quicksort
  • Shell sort
  • Counting sort

Q 10. Print: 

  • Each path from the root to leaf in a binary tree
  • The vertical sum of a binary tree
  1. In a graph, write an algorithm to do the following:
  • Depth-first search
  • Breadth-first search

Q11. Implement the following algorithms in Java:

  • Dijkstra algorithm
  • Bellman-Ford algorithm
  • Kruskal’s algorithm 
  1. Given two strings, find the longest common substring.
  2. Given an MxN matrix, where you can only move down or right, count all paths from top left to bottom right.

FAQs on Java Algorithms Interview Questions 

Q1. What algorithms should I know for Java interviews?

You should definitely know algorithms with low time complexity, like O(1), O(logN), and O(N*logN), that also don’t take up too much space for your Java interviews. More importantly, you should know how to implement them.

Q2. Why are algorithm questions asked in interviews?

Algorithms are a crucial part of any software development process. An efficient algorithm can run for millions of users in a short time, taking little space and using only a reasonable amount of system resources. An inefficient or inaccurate algorithm can break, cause a company a significant loss, and wastes time, space, and other system resources. Hence, algorithm skills are important in interviews.

Q3. What makes an algorithm greedy?

An algorithm that uses the strategy of making the most optimal choice at each small step with the hope that this will eventually lead to a globally optimal solution is a greedy algorithm.

Q4. What are some different types of algorithms?

Simple recursive, backtracking, divide and conquer, dynamic programming, greedy, branch and bound, brute force, and randomized algorithms are some different types of algorithms.

Q5. What are five things algorithms must possess?

The five things algorithms must possess are definiteness, inputs, outputs, finiteness, and effectiveness.

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