DSA Mini Q&A Module – Arrays, ArrayList, Linked List, and Stack

DSA Mini Q&A Module

1. Arrays

Q1: What is an array?

An array is a collection of elements of the same type stored in contiguous memory locations. Each element is accessed by an index starting from 0.

Q2: Advantages of arrays

• Fast access via index (O(1))
• Easy to iterate

Q3: Disadvantages of arrays

• Fixed size (cannot grow/shrink dynamically)
• Inserting/deleting in middle is expensive (O(n))

Q4: Array vs ArrayList

Feature	Array	ArrayList
Size	Fixed	Dynamic
Data type	Same type (primitive or object)	Objects only
Memory	Contiguous	Flexible, resizable
Performance	Faster for access	Slightly slower due to dynamic resizing

2. ArrayList

Q1: What is an ArrayList?

A resizable array in Java, part of java.util, which can grow automatically and store objects.

Q2: Pros over array

• Dynamic size
• Built-in methods (add(), remove(), contains())

Q3: Cons

• Slower than array for primitive types (stores objects, autoboxing overhead)
• Not synchronized (use Vector if you need thread safety)

3. Linked List

Q1: What is a linked list?

A linear data structure where each element (node) contains data + pointer to next node.

Types

• Singly Linked List – each node points to next node
• Doubly Linked List – nodes have next and previous pointers
• Circular Linked List – last node points to first node, can be singly or doubly

Q2: Pros & Cons

Feature	Linked List	Array
Dynamic size	Yes	No
Insert/delete	Fast (O(1) if node known)	Slow (O(n))
Random access	Slow (O(n))	Fast (O(1))
Memory	Extra pointer memory	Contiguous, less overhead

Q3: When is a doubly linked list better?

When you need reverse traversal or easier deletion of a node if you have a pointer to it.

Q4: Circular vs Singly

Circular is better for queues or round-robin tasks, as you can loop back to start easily.

4. Stack

Q1: What is a stack?

A LIFO (Last-In-First-Out) data structure. Operations include push() → add element, pop() → remove top element, peek() → see top element.

Q2: Implementation

• Array-based – fixed size, fast access, O(1) push/pop if not full
• Linked List-based – dynamic size, memory overhead for pointers, O(1) push/pop

Q3: Array vs Linked List Stack

Feature	Array	Linked List
Size	Fixed	Dynamic
Memory	Contiguous	Pointer overhead
Push/Pop	O(1)	O(1)
Wasted space	Possible if not full	None

Q4: Why use a stack?

• Expression evaluation (infix, postfix, prefix)
• Undo operations in editors
• Function call stack (recursion)

5. Conceptual “Why/When” Questions

• Why choose ArrayList over Array? → Dynamic size and frequent insertions/deletions
• Why choose LinkedList over ArrayList? → Frequent insertions/deletions in the middle
• Why choose doubly linked list over singly? → Reverse traversal or easier deletion
• Why array stack over linked list stack? → Simple, fast, less memory overhead when size is known
• Why linked list stack over array stack? → Dynamic size, no overflow

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