Which data structures are used for BFS and DFS of a graph?

A Queue is used for BFS
A Stack is used for DFS. DFS can also be implemented using recursion.

Which Data Structure Should be used for implementing LRU cache?

We use two data structures to implement an LRU Cache.

A queue which is implemented using a doubly linked list. The maximum size of the queue will be equal to the total number of frames available (cache size). The most recently used pages will be near rear end and least recently pages will be near the front end.
A Hash with page number as key and address of the corresponding queue node as value.

How to check if a given Binary Tree is BST or not?

If inorder traversal of a binary tree is sorted, then the binary tree is BST. The idea is to simply do inorder traversal and while traversing keep track of previous key value. If current key value is greater, then continue, else return false. See A program to check if a binary tree is BST or not for more details.

What are linear and nonlinear data Structures?

Linear: A data structure is said to be linear if its elements form a sequence or a linear list. Examples: Array. Linked List, Stacks and Queues
Non-Linear: A data structure is said to be non-linear if traversal of nodes is nonlinear in nature. Example: Graph and Trees.

What are the various operations that can be performed on different Data Structures?

Insertion: Add a new data item in the given collection of data items.
Deletion: Delete an existing data item from the given collection of data items.
Traversal: Access each data item exactly once so that it can be processed.
Searching: Find out the location of the data item if it exists in the given collection of data items.
Sorting: Arranging the data items in some order i.e. in ascending or descending order in case of numerical data and in dictionary order in case of alphanumeric data.

What is BFS/DFS for a Graph?

Breadth First Traversal (or Search) for a graph is similar to Breadth First Traversal of a tree. Unlike trees, graphs contain cycles, so we may come to the same node again. To avoid processing a node more than once, we use a boolean visited array. For simplicity, it is assumed that all vertices are reachable from the starting vertex.

Depth First Traversal (or Search) for a graph is similar to Depth First Traversal of a tree. Unlike trees, graphs contain cycles, so we may come to the same node again. To avoid processing a node more than once, we use a boolean visited array.

Explain what is Skip list?

Skip list the method for data structuring, where it allows the algorithm to search, delete and insert elements in a symbol table or dictionary. In a skip list, each element is represented by a node. The search function returns the content of the value related to key. The insert operation associates a specified key with a new value, while the delete function deletes the specified key.

What is the Difference between ArrayList vs HashMap in Java?

1) ArrayList implements List interface while HashMap implements Map interface in Java.
2) ArrayList only stores one object while HashMap stores two objects key and value.
3) Keys of HashMap must implement equals and hashCode method correctly, ArrayList doesn’t have that requirement but its good to have that because contains() method of ArrayList will use equals() method to see if that object already exists or not.
4) ArrayList maintains the order of object, in which they are inserted while HashMap doesn’t provide any ordering guarantee.
5) ArrayList allows duplicates but HashMap doesn’t allow duplicates key though it allows duplicate values.
6) ArrayList get(index) method always gives an O(1) performance but HashMap get(key) can be O(1) in the best case and O(n) in the worst case.

What is a  slow pointer and fast pointer?

Slow pointer and fast pointer are simply the names given to two pointer variables. The only difference is that, slow pointer travels the linked list one node at a time whereas a fast pointer travels the linked list two nodes at a time.
Given below is a basic code snippet for moving slow and fast pointers.


struct node *slow_ptr, *fast_ptr;

while(!slow_ptr && !fast_ptr && fast_ptr->next != NULL) {
slow_ptr = slow_ptr->next; // moves one node ahead at a time
fast_ptr = fast_ptr->next->next; // moves two nodes ahead at a time


This concept can be used in cases like detecting a loop in a graph, finding the middle node of a linked list (better time complexity), flattening a linked list etc. All these examples use the idea of slow and fast pointers.


private static boolean isLoopPresent(Node startNode){
Node slowPointer = startNode; // Initially ptr1 is at starting location.
Node fastPointer = startNode; // Initially ptr2 is at starting location.

while(fastPointer!=null && fastPointer.getNext()!=null){ // If ptr2 encounters NULL, it means there is no Loop in Linked list.
slowPointer = slowPointer.getNext(); // ptr1 moving one node at at time
fastPointer = fastPointer.getNext().getNext(); // ptr2 moving two nodes at at time

if(slowPointer==fastPointer) // if ptr1 and ptr2 meets, it means linked list contains loop.
return true;


How is an Array different from Linked List?

The size of the arrays is fixed, Linked Lists are Dynamic in size.
Inserting and deleting a new element in an array of elements is expensive, Whereas both insertion and deletion can easily be done in Linked Lists.
Random access is not allowed in Linked Listed.
Extra memory space for a pointer is required with each element of the Linked list.
Arrays have better cache locality that can make a pretty big difference in performance.

What is a Queue, how it is different from a stack and how is it implemented?

A queue is a linear structure which follows the order is First In First Out (FIFO) to access elements. Mainly the following are basic operations on queue: Enqueue, Dequeue, Front, Rear
The difference between stacks and queues is in removing. In a stack we remove the item the most recently added; in a queue, we remove the item the least recently added. Both Queues and Stacks can be implemented using Arrays and Linked Lists.

What is a HashTable, and what is the average case and worst case time for each of its operations? How can we use this structure to find all anagrams in a dictionary?

A Hash Table is a data structure for mapping values to keys of arbitrary type. The Hash Table consists of a sequentially enumerated array of buckets of a certain length. We assign each possible key to a bucket by using a hash function – a function that returns an integer number (the index of a bucket) for any given key. Multiple keys can be assigned to the same bucket, so all the (key, value) pairs are stored in lists within their respective buckets.

Choosing the right hashing function can have a great impact on performance. A hash function that is good for a dataset that we want to store will result in hashes of different keys being a rare occurrence. Even though accessing, inserting and deleting have a worst-case time complexity of O(N) (where N is the number of elements in the Hash Table), in practice we have an average time complexity of O(1).

To find all anagrams in a dictionary, we just have to group all words that have the same set of letters in them. So, if we map words to strings representing their sorted letters, we could group words into lists by using their sorted letters as a key.

The hash table stores lists mapped to strings. For each word, we add it to the list at the appropriate key, or make a new list and add it to it then. On average, for a dictionary of N words of length less or equal to L, this algorithm works with an average time complexity of O(N L log L).

What are Red-Black Trees and B-Trees? What is the best use case for each of them?

Both Red-Black Trees and B-Trees are balanced search trees that can be used on items that can have a comparison operator defined on them. They allow operations like minimum, maximum, predecessor, successor, insert, and delete, in O(log N) time (with N being the number of elements). Thus, they can be used for implementing a map, priority queue, or index of a database, to name a few examples.

Red-Black Trees are an improvement upon Binary Search Trees. Binary Search Trees use binary trees to perform the named operations, but the depth of the tree is not controlled – so operations can end up taking a lot more time than expected. Red-Black Trees solve this issue by marking all nodes in the tree as red or black, and setting rules of how certain positions between nodes should be processed. Without going into too much detail, this method guarantees that the longest branch isn’t more than twice as long as the shortest branch, so each branch is shorter than 2*log_base2(N).

This is the ideal structure for implementing ordered maps and priority queues. B-Trees branch into K-2K branches (for a given number K) rather than into 2, as is typical for binary trees. Other than that, they behave pretty similarly to a binary search tree. This has the advantage of reducing access operations, which is particularly useful when data is stored on secondary storage or in a remote location. This way, we can request data in larger chunks, and by the time we finish processing a previous request, our new request is ready to be handled. This structure is often used in implementing databases, since they have a lot of secondary storage access.

What are Infix, prefix, Postfix notations?

Infix notation: X + Y – Operators are written in-between their operands. This is the usual way we write expressions. An expression such as
A * ( B + C ) / D
Postfix notation (also known as “Reverse Polish notation”): X Y + Operators are written after their operands. The infix expression given above is equivalent to
A B C + * D/
Prefix notation (also known as “Polish notation”): + X Y Operators are written before their operands. The expressions given above are equivalent to
/ * A + B C D