Top 15 Data Structures and Algorithm Interview Questions for Java programmer - Answers

Data structures and algorithm questions are important part of any
programming job interview, be it a Java interview, C++ interview or any other
programming language. Since data structures is core programming concept, its
mandatory for all programmers, to know basic data structures like stack, linked
list, queue, array, tree and graph. Though tree and graph are on tough side, I
still see programmers to get familiar will all these. Any list of programming job interview questions
is incomplete without questions from data structures and algorithms. Similarly while
going on questions from data structure you may get some programming exercise as
well e.g. swapping numbers without temp variable
. Linked
list and arrays are favorite topics in any data structure interview, questions
like reversing linked list, traversing linked list or deleting nodes from
linked list, which involves algorithm and data structures are quite common.
Similarly, finding duplicates in array, finding missing numbers, sorting arrays
are very popular. You can also expect questions from stack, queue, array,
linked list, tree, graph and HashMap are most common in any data structure
interview. In this tutorial,  we will see
couple of data structure questions answers from these topic. Let us know, if
you have any interesting questions from data structures and algorithm, which
you faced during any Java interviews.

Data Structures and Algorithm Interview
Questions for Java Programmers

This is combined list of questions from various data structure e.g.
array, linked list, stack or queue. It includes some coding questions as well,
which gel with data structures. 

Question
1 : How to find middle element of linked list in one pass?

One of the most popular question from data structures and algorithm,
mostly asked on telephonic interview. Since many programmer know that, in order
to find length of linked list we need to first traverse through linked list till
we find last node, which is pointing to null, and then in second pass we can
find middle element by traversing only half of length. They get confused when
interviewer ask him to do same job in one pass. In order to find middle element
of linked list in one pass you need to maintain two pointer, one increment at
each node while other increments after two nodes at a time, by having this
arrangement, when first pointer reaches end, second pointer will point to
middle element of linked list. See this trick to find middle element of linked list in
single pass for more details.

Question
2 : How to find if linked list has loop ?

This question has bit of similarity with earlier algorithm and data
structure interview question. I mean we can use two pointer approach to solve
this problem. If we maintain two pointers, and we increment one pointer after
processing two nodes and other after processing every node, we are likely to
find a situation where both the pointers will be pointing to same node. This
will only happen if linked list has loop.

Question
3 : How to find 3rd element from end in a linked list in one pass?

This is another frequently asked linked list interview question. This
question is exactly similar to finding middle element of linked list in
single pass. If we apply same trick of maintaining two pointers and
increment other pointer, when first has moved upto 3rd element, than when first
pointer reaches to the end of linked list, second pointer will be pointing to
the 3rd element from last in a linked list.

Question
4 : In an integer array, there is 1 to 100 number, out of one is duplicate, how to find ?

This is a rather simple data structures question, especially for this
kind of. In this case you can simply add all numbers stored in array, and total
sum should be equal to n(n+1)/2. Now just subtract actual sum to
expected sum, and that is your duplicate number. Of course there is a brute
force way of checking each number against all other numbers, but that will
result in performance of O(n^2) which is not good. By the way this trick will
not work if array have multiple duplicates or its not numbers forming
arithmetic progression. Here is example of one way to find duplicate number in array.

Question
6 : How to reverse String in Java ?

This is one of my favorite question. Since String is one of
the most important type in programming, you expect lot of question related to
String any data structure interview. There are many ways to reverse Sting in
Java or any other programming language, and interviewer will force you to solve
this problem by using without API i.e. without using reverse() method of StringBuffer. In
follow-up he may ask to reverse String using recursion as well. See 3 ways to reverse String in Java
to learn reversing String using both loops and recursion in Java.

Question
7 : Write a Java program to sort a array using Bubble Sort algorithm?

I have always send couple of questions from searching and sorting in data
structure interviews. Bubble sort is one of the simplest sorting algorithm but
if you ask anyone to implement on the spot it gives you an opportunity to gauge
programming skills of a candidate. See How to sort array using Bubble Sort in
Java for complete solution of this datastrucutre interview question.

Question
8 : What is difference between Stack and Queue data structure ?

One of the classical datastrucutre interview question. I guess every one
know, No? Any way main difference is that Stack is LIFO(Last In First Out) data
structure while Queue is a FIFO(First In First Out) data structure.

Question
9 : How do you find duplicates in array if there is more than one duplicate?

Sometime this is asked as follow-up question of earlier datastrucutre
interview question, related to finding duplicates in Array. One way of solving
this problem is using a Hashtable or HashMap data structure.
You can traverse through array, and store each number as key and number of
occurrence as value. At the end of traversal you can find all duplicate numbers,
for which occurrence is more than one. In Java if a number already exists in HashMap then calling get(index) will
return number otherwise it return null. this property can be used to insert or
update numbers in HashMap.

Question
10 : What is difference between Singly Linked List and Doubly Linked List data
structure?

This is another classical interview question on data structure, mostly
asked on telephonic rounds. Main difference between singly linked list and doubly
linked list is ability to traverse. In a single linked list, node only points
towards next node, and there is no pointer to previous node, which means you
can not traverse back on a singly linked list. On the other hand doubly linked list maintains two pointers, towards next and previous node, which allows
you to navigate in both direction in any linked list.

Question
11 : Write Java program to print Fibonacci series ?

This is not a data structures question, but a programming one, which many
times appear during data structure interview. Fibonacci series is a mathematical
series, where each number is sum of previous two numbers e.g. 1,1, 2, 3, 5, 8,
13, 21. Interviewer is often interested in two things, a function which returns
nth number in Fibonacci series and solving this problem using recursion in
Java. Though, its easy question, recursion part often confuses beginners. See
this link to find nth Fibonacci number in Java.

Question
12 : Write Java program to check if a number is palindrome or not?

This is similar to previous question, not directly related to data
structures, but quite popular along with other questions. A number is called
palindrome, if reverse of number is equal to number itself. Interviewer ask to
solve this problem without taking help from Java API or any open source
library. Any way it’s simple question, you can use division operator (/) and
remainder operator (%) to solve this question. Just remember, division operator
can be used to get rid of last digit e.g. 1234/10 will give you 123, and
modulus operator can give you last digit e.g. 1234%10 will return 4. By the
way, here is a Java program check if number is
palindrome or not.

Question
13 : What is binary search tree?

This is a data structure question from Tree data structures. Binary
Search Tree has some special properties e.g. left nodes contains items whose
value is less than root , right sub tree contains keys with higher node value
than root, and there should not be any duplicates in the tree. Apart from
definition, interview can ask you to implement binary search tree in Java and questions
on tree traversal e.g. IN order, preorder, and post order traversals are quite
popular data structure question.

Question
14 : How to reverse linked list using recursion and iteration?

This is another good question on data structures. There are many
algorithm to reverse linked list and you can search of them using google. I am
thinking of writing another blog post to explain linked list reversal and will
share with you later.

Question
15 : Write a Java program to implement Stack in Java?

You can implement Stack by using array or linked list. This question
expect you to implement standard method provided by stack data structure e.g. push() and pop().  Both push() and pop() should be
happen at top of stack, which you need to keep track. It’s also good if you can
implement utility methods like contains(), isEmpty() etc. By
the way JDK has java.util.Stack class and you can check it’s code to get an
idea. You can also check Effective Java book, where
Josh Bloch has explains how an incorrect implementation of stack can cause memory leak in Java.

That's all on this list of data structure interview questions and
answers. This is one topic, which is always asked in any programming interview,
doesn't matter if you are C, C++, or Java developer, basic knowledge of data
structure like array, linked list, stack, queue, tree are must to clear any
programming interview.

Source:http://javarevisited.blogspot.com/2013/03/top-15-data-structures-algorithm-interview-questions-answers-java-programming.html