Data Structures Interview Questions Set -1 
 
What is data structure?
A data structure is a way of organizing data that considers 
not only the items stored, but also their relationship to 
each other. Advance knowledge about the relationship 
between data items allows designing of efficient algorithms 
for the manipulation of data.
List out the areas in which data structures are applied 
extensively?
Compiler Design, Operating System, Database Management 
System, Statistical analysis package, Numerical Analysis, 
Graphics, Artificial Intelligence, Simulation
If you are using C language to implement the heterogeneous 
linked list, what pointer type will you use?
The heterogeneous linked list contains different data types 
in its nodes and we need a link, pointer to connect them. 
It is not possible to use ordinary pointers for this. So we 
go for void pointer. Void pointer is capable of storing 
pointer to any type as it is a generic pointer type.
What is the data structures used to perform recursion?
Stack. Because of its LIFO (Last In First Out) property it 
remembers its caller, so knows whom to return when the 
function has to return. Recursion makes use of system stack 
for storing the return addresses of the function calls. 
Every recursive function has its equivalent iterative (non-
recursive) function. Even when such equivalent iterative 
procedures are written, explicit stack is to be used.
What are the methods available in storing sequential files ?
Straight merging, Natural merging,  Polyphase sort, 
Distribution of Initial runs.
List out few of the Application of tree data-structure?
The manipulation of Arithmetic expression, Symbol Table 
construction, Syntax analysis.
In RDBMS, what is the efficient data structure used in the 
internal storage representation?
B+ tree. Because in B+ tree, all the data is stored only in 
leaf nodes, that makes searching easier. This corresponds 
to the records that shall be stored in leaf nodes.
What is a spanning Tree?
A spanning tree is a tree associated with a network. All 
the nodes of the graph appear on the tree once. A minimum 
spanning tree is a spanning tree organized so that the 
total edge weight between nodes is minimized.
Does the minimum spanning tree of a graph give the shortest 
distance between any 2 specified nodes?
Minimal spanning tree assures that the total weight of the 
tree is kept at its minimum. But it doesn't mean that the 
distance between any two nodes involved in the minimum-
spanning tree is minimum.
Whether Linked List is linear or Non-linear data structure?
According to Access strategies Linked list is a linear one. 
According to Storage Linked List is a Non-linear one.
What is the quickest sorting method to use?
The answer depends on what you mean by quickest. For most 
sorting problems, it just doesn't matter how quick the sort 
is because it is done infrequently or other operations take 
significantly more time anyway. Even in cases in which 
sorting speed is of the essence, there is no one answer. It 
depends on not only the size and nature of the data, but 
also the likely order. No algorithm is best in all cases. 
There are three sorting methods in this author's toolbox 
that are all very fast and that are useful in different 
situations. Those methods are quick sort, merge sort, and 
radix sort.

The Quick Sort
The quick sort algorithm is of the divide and conquer type. 
That means it works by reducing a sorting problem into 
several easier sorting problems and solving each of them. A 
dividing value is chosen from the input data, and the data 
is partitioned into three sets: elements that belong before 
the dividing value, the value itself, and elements that 
come after the dividing value. The partitioning is 
performed by exchanging elements that are in the first set 
but belong in the third with elements that are in the third 
set but belong in the first Elements that are equal to the 
dividing element can be put in any of the three sets the 
algorithm will still work properly.

The Merge Sort
The merge sort is a divide and conquer sort as well. It 
works by considering the data to be sorted as a sequence of 
already-sorted lists (in the worst case, each list is one 
element long). Adjacent sorted lists are merged into larger 
sorted lists until there is a single sorted list containing 
all the elements. The merge sort is good at sorting lists 
and other data structures that are not in arrays, and it 
can be used to sort things that don't fit into memory. It 
also can be implemented as a stable sort.

The Radix Sort
The radix sort takes a list of integers and puts each 
element on a smaller list, depending on the value of its 
least significant byte. Then the small lists are 
concatenated, and the process is repeated for each more 
significant byte until the list is sorted. The radix sort 
is simpler to implement on fixed-length data such as ints.
How can I search for data in a linked list?
Unfortunately, the only way to search a linked list is with 
a linear search, because the only way a linked list's 
members can be accessed is sequentially. Sometimes it is 
quicker to take the data from a linked list and store it in 
a different data structure so that searches can be more 
efficient.
What is the heap?
The heap is where malloc(), calloc(), and realloc() get 
memory.

Getting memory from the heap is much slower than getting it 
from the stack. On the other hand, the heap is much more 
flexible than the stack. Memory can be allocated at any 
time and deallocated in any order. Such memory isn't 
deallocated automatically; you have to call free().
Recursive data structures are almost always implemented 
with memory from the heap. Strings often come from there 
too, especially strings that could be very long at runtime. 
If you can keep data in a local variable (and allocate it 
from the stack), your code will run faster than if you put 
the data on the heap. Sometimes you can use a better 
algorithm if you use the heap faster, or more robust, or 
more flexible. Its a tradeoff.
If memory is allocated from the heap, its available until 
the program ends. That's great if you remember to 
deallocate it when you're done. If you forget, it's a 
problem. A �memory leak is some allocated memory that's no 
longer needed but isn't deallocated. If you have a memory 
leak inside a loop, you can use up all the memory on the 
heap and not be able to get any more. (When that happens, 
the allocation functions return a null pointer.) In some 
environments, if a program doesn't deallocate everything it 
allocated, memory stays unavailable even after the program 
ends.
What is the easiest sorting method to use?
The answer is the standard library function qsort(). It's 
the easiest sort by far for several reasons:
It is already written.
It is already debugged.
It has been optimized as much as possible (usually).
Void qsort(void *buf, size_t num, size_t size, int (*comp)
(const void *ele1, const void *ele2));




Data Structures Interview Questions Set -2
 
What is the bucket size, when the overlapping and collision 
occur at same time?
One. If there is only one entry possible in the bucket, 
when the collision occurs, there is no way to accommodate 
the colliding value. This results in the overlapping of 
values.
In an AVL tree, at what condition the balancing is to be 
done?
If the pivotal value (or the Height factor) is greater than 
1 or less than 1.
Minimum number of queues needed to implement the priority 
queue?
Two. One queue is used for actual storing of data and 
another for storing priorities.
How many different trees are possible with 10 nodes ?
1014 - For example, consider a tree with 3 nodes(n=3), it 
will have the maximum combination of 5 different (ie, 23 - 
3 =? 5) trees.
What is a node class?
A node class is a class that, relies on the base class for 
services and  implementation, provides a wider interface to 
users than its base class, relies primarily on virtual 
functions in its public interface depends on all its direct 
and indirect base class can be understood only in the 
context of the base class can be used as base for further 
derivation
can be used to create objects. A node class is a class that 
has added new services or functionality beyond the services 
inherited from its base class.
When can you tell that a memory leak will occur?
A memory leak occurs when a program loses the ability to 
free a block of dynamically allocated memory.
What is placement new?
When you want to call a constructor directly, you use the 
placement new. Sometimes you have some raw memory that’s 
already been allocated, and you need to construct an object 
in the memory you have. Operator new’s special version 
placement new allows you to do it.
class Widget
{
public :
Widget(int widgetsize);
…
Widget* Construct_widget_int_buffer(void *buffer,int 
widgetsize)
{
return new(buffer) Widget(widgetsize);
}
};
This function returns a pointer to a Widget object that’s 
constructed within the buffer passed to the function. Such 
a function might be useful for applications using shared 
memory or memory-mapped I/O, because objects in such 
applications must be placed at specific addresses or in 
memory allocated by special routines.
List out the areas in which data structures are applied 
extensively ?
Compiler Design, Operating System, Database Management 
System, Statistical analysis package, Numerical Analysis, 
Graphics, Artificial Intelligence, Simulation
If you are using C language to implement the heterogeneous 
linked list, what pointer type will you use?
The heterogeneous linked list contains different data types 
in its nodes and we need a link, pointer to connect them. 
It is not possible to use ordinary pointers for this. So we 
go for void pointer. Void pointer is capable of storing 
pointer to any type as it is a generic pointer type.
What is the data structures used to perform recursion?
Stack. Because of its LIFO (Last In First Out) property it 
remembers its caller so knows whom to return when the 
function has to return. Recursion makes use of system stack 
for storing the return addresses of the function calls. 
Every recursive function has its equivalent iterative (non-
recursive) function. Even when such equivalent iterative 
procedures are written, explicit stack is to be used.
Whether Linked List is linear or Non-linear data structure?
According to Access strategies Linked list is a linear one.

According to Storage Linked List is a Non-linear one
Tell how to check whether a linked list is circular ?
Create two pointers, each set to the start of the list. 
Update each as follows:

while (pointer1)

{
pointer1 = pointer1->next;
pointer2 = pointer2->next; if (pointer2) pointer2=pointer2-
>next;
if (pointer1 == pointer2)

? ? ? ? ? ? {
print (\”circular\n\”);
}
}
What is the difference between ARRAY and STACK?
STACK follows LIFO. Thus the item that is first entered 
would be the last removed.

In array the items can be entered or removed in any order. 
Basically each member access is done using index. No strict 
order is to be followed here to remove a particular element.
What is the difference between NULL AND VOID pointer?
NULL can be value for pointer type variables.
VOID is a type identifier which has not size.
NULL and void are not same. Example: void* ptr = NULL;
What is precision?
Precision refers the accuracy of the decimal portion of a 
value. Precision is the number of digits allowed after the 
decimal point.
What is impact of signed numbers on the memory?
Sign of the number is the first bit of the storage 
allocated for that number. So you get one bit less for 
storing the number. For example if you are storing an 8-bit 
number, without sign, the range is 0-255. If you decide to 
store sign you get 7 bits for the number plus one bit for 
the sign. So the range is -128 to +127.
How memory is reserved using a declaration statement ?
Memory is reserved using data type in the variable 
declaration. A programming language implementation has 
predefined sizes for its data types.

For example, in C# the declaration int i; will reserve 32 
bits for variable i.

A pointer declaration reserves memory for the address or 
the pointer variable, but not for the data that it will 
point to. The memory for the data pointed by a pointer has 
to be allocated at runtime.

The memory reserved by the compiler for simple variables 
and for storing pointer address is allocated on the stack, 
while the memory allocated for pointer referenced data at 
runtime is allocated on the heap.
How many parts are there in a declaration statement?





 


Data Structures Interview Questions Set -3
 
There are two main parts, variable identifier and data type 
and the third type is optional which is type qualifier like 
signed/unsigned.
Is Pointer a variable?
Yes, a pointer is a variable and can be used as an element 
of a structure and as an attribute of a class in some 
programming languages such as C++, but not Java. However, 
the contents of a pointer is a memory address of another 
location of memory, which is usually the memory address of 
another variable, element of a structure, or attribute of a 
class.
What is Data Structure?
A data structure is a group of data elements grouped 
together under one name. These data elements, known as 
members, can have different types and different lengths. 
Some are used to store the data of same type and some are 
used to store different types of data.
What is significance of  ” * ” ?
The symbol “*” tells the computer that you are declaring a 
pointer.
Actually it depends on context.
In a statement like int *ptr; the ‘*’ tells that you are 
declaring a pointer.
In a statement like int i = *ptr; it tells that you want to 
assign value pointed to by ptr to variable i.

The symbol “*” is also called as Indirection Operator/ 
Dereferencing Operator.
Why do we Use a Multidimensional Array?
A multidimensional array can be useful to organize 
subgroups of data within an array. In addition to 
organizing data stored in elements of an array, a 
multidimensional array can store memory addresses of data 
in a pointer array and an array of pointers.

Multidimensional arrays are used to store information in a 
matrix form.
e.g. a railway timetable, schedule cannot be stored as a 
single dimensional array.
One can use a 3-D array for storing height, width and 
length of each room on each floor of a building.
How do you assign an address to an element of a pointer 
array ?
We can assign a memory address to an element of a pointer 
array by using the address operator, which is the ampersand 
(&), in an assignment statement such as ptemployee[0] = 
&projects[2];
Run Time Memory Allocation is known as ?
Allocating memory at runtime is called a dynamically 
allocating memory. In this, you dynamically allocate memory 
by using the new operator when declaring the array, for 
example : int grades[] = new int[10];
What method is used to place a value onto the top of a 
stack?
push() method, Push is the direction that data is being 
added to the stack. push() member method places a value 
onto the top of a stack.
What method removes the value from the top of a stack?
The pop() member method removes the value from the top of a 
stack, which is then returned by the pop() member method to 
the statement that calls the pop() member method.
What does isEmpty() member method determines?
isEmpty() checks if the stack has at least one element. 
This method is called by Pop() before retrieving and 
returning the top element.
What is a queue ?
A Queue is a sequential organization of data. A queue is a 
first in first out type of data structure. An element is 
inserted at the last position and an element is always 
taken out from the first position.
What is the relationship between a queue and its underlying 
array?
Data stored in a queue is actually stored in an array. Two 
indexes, front and end will be used to identify the start 
and end of the queue.

When an element is removed front will be incremented by 1. 
In case it reaches past the last index available it will be 
reset to 0. Then it will be checked with end. If it is 
greater than end queue is empty.

When an element is added end will be incremented by 1. In 
case it reaches past the last index available it will be 
reset to 0. After incrementing it will be checked with 
front. If they are equal queue is full.
Which process places data at the back of the queue?
Enqueue is the process that places data at the back of the 
queue.
Why is the isEmpty() member method called?
The isEmpty() member method is called within the dequeue 
process to determine if there is an item in the queue to be 
removed i.e. isEmpty() is called to decide whether the 
queue has at least one element. This method is called by 
the dequeue() method before returning the front element.
How is the front of the queue calculated ?
The front of the queue is calculated by front = (front+1) % 
size.
What does each entry in the Link List called?
Each entry in a linked list is called a node. Think of a 
node as an entry that has three sub entries. One sub entry 
contains the data, which may be one attribute or many 
attributes. Another points to the previous node, and the 
last points to the next node. When you enter a new item on 
a linked list, you allocate the new node and then set the 
pointers to previous and next nodes.
What is Linked List ?
Linked List is one of the fundamental data structures. It 
consists of a sequence of? nodes, each containing arbitrary 
data fields and one or two (”links”) pointing to the next 
and/or previous nodes. A linked list is a self-referential 
datatype because it contains a pointer or link to another 
data of the same type. Linked lists permit insertion and 
removal of nodes at any point in the list in constant time, 
but do not allow random access.
What member function places a new node at the end of the 
linked list?
The appendNode() member function places a new node at the 
end of the linked list. The appendNode() requires an 
integer representing the current data of the node.
How is any Data Structure application is classified among 
files?
A linked list application can be organized into a header 
file, source file and main application file. The first file 
is the header file that contains the definition of the NODE 
structure and the LinkedList class definition. The second 
file is a source code file containing the implementation of 
member functions of the LinkedList class. The last file is 
the application file that contains code that creates and 
uses the LinkedList class.
Which file contains the definition of member functions?
Definitions of member functions for the Linked List class 
are contained in the LinkedList.cpp file.
What are the major data structures used in the following 
areas : RDBMS, Network data model & Hierarchical data model.
1. RDBMS Array (i.e. Array of structures)
2. Network data model Graph
3. Hierarchical data model Trees.
Difference between calloc and malloc ?
malloc: allocate n bytes
calloc: allocate m times n bytes initialized to 0