Stacks & Queues: Concepts, Applications, and Interview Questions

Welcome to the Null Pointer Club Newsletter! In my career, I would have taken more than 500 interviews across all developer roles. One data structure which was part of every interview is Stacks or Queues.

Interviewers often favor data structures like stacks and queues because they effectively assess your problem-solving abilities and your grasp of memory management. These straightforward yet powerful structures are fundamental to many algorithms, ranging from backtracking to task scheduling. Let’s delve into their details, examine their practical applications, and address common interview questions!

Understanding Stacks & Queues

Stack (LIFO - Last In, First Out)

A stack is a linear data structure that adheres to the LIFO principle. This means that the last element added is the first one to be removed. Imagine a stack of plates – you always take from the top and add to the top.

Common Operations:

• push(x): Add an element x to the stack.

• pop(): Remove the top element.

• peek()/top(): View the top element without removing it.

• isEmpty(): Check if the stack is empty.

Real-World Applications:

• Undo/Redo functionality in text editors

• Backtracking (e.g., solving mazes, recursion calls)

• Expression evaluation (infix to postfix conversion, syntax parsing)

Queue (FIFO - First In, First Out)

A queue is a linear data structure that follows the FIFO principle. The first element added is the first to be removed. Think of people standing in a queue – the first person in line is served first.

Common Operations:

• enqueue(x): Add an element x to the rear.

• dequeue(): Remove the front element.

• front(): View the front element without removing it.

• isEmpty(): Check if the queue is empty.

Real-World Applications:

• Task scheduling in operating systems (CPU scheduling, print queue)

• Managing requests in web servers

• Breadth-first search (BFS) in graphs

1. Implement a Stack Using an Array or Linked List

• Use an array for a fixed-size stack, or a linked list for a dynamic stack.

• Keep a top pointer to track the last element.

2. Implement a Queue Using Stacks

• Use two stacks (one for enqueue, one for dequeue) to simulate FIFO behavior.

• For dequeue operation, move all elements from the enqueue stack to the dequeue stack if it’s empty.

3. Check for Balanced Parentheses in an Expression

• Use a stack to push opening brackets and pop when a matching closing bracket appears.

• If the stack is empty at the end, the expression is balanced.

4. Find the Next Greater Element Using a Stack

• Traverse the array from right to left.

• Maintain a stack of elements and check if the current element is smaller than the stack’s top.

• Pop smaller elements until you find a greater element.

5. Implement a Circular Queue

• Use an array with two pointers (front and rear).

• Handle wrap-around cases to ensure space is utilized efficiently.

6. Design a Min Stack (Stack with Constant Time Min Retrieval)

• Maintain an additional stack to track minimum elements.

• Push to the min stack only when a new minimum appears.

7. Find the First Non-Repeating Character in a Stream

• Use a queue to maintain the order of characters and a hash map to track frequencies.

• As characters arrive, update the queue and hash map accordingly.

Tips & Extra Notes

1. Understand When to Use a Stack vs. Queue

• Use stacks when you need to reverse an order (e.g., DFS, recursion, backtracking).

• Use queues when you need to process items in order (e.g., BFS, scheduling).

2. Master Edge Cases

• Handling empty stacks/queues.

• Implementing efficient space usage in circular queues.

3. Optimize Your Solutions

• Use deque in Python (collections.deque) instead of list for O(1) queue operations.

• Use priority queues (heaps) when needed for ordered queue processing.

Basic data structures like stacks and queues are commonly seen in coding interviews. Gaining a thorough understanding of their ideas, applications, and typical interview questions will equip you to confidently take on technological obstacles.

Practice Problem: Try implementing a queue using only one stack! Can you do it efficiently?

— Null Pointer Club Team