Stacks & Queues: Beginner

LIFO or FIFO? Pick the right lane and the problem solves itself.

LIFO or FIFO? Pick the right lane and the problem solves itself.

Category

Python

Difficulty

beginner

Duration

25 minutes

Challenges

0 hands-on challenges

Topics covered: Stack Fundamentals: The LIFO Discipline, Balanced Parentheses: The Stack Problem, Queue Fundamentals: The FIFO Discipline, Monotonic Stack: The Next Greater Element, Common Bugs and Python Pitfalls

Lesson Sections

1. Stack Fundamentals: The LIFO Discipline (concepts: pyStackFundamentals, pyLIFO)

   A stack is a pile. You put things on top, you take things from the top. That is LIFO: last in, first out. The book you just placed on the pile is the first one you can grab. In Python, a plain list works perfectly as a stack. append() pushes to the top in O(1). pop() removes from the top in O(1). There is no stack class in Python's standard library because list already nails it. When the Stack Is the Right Tool list vs collections.deque for Stacks In an interview, say: 'I will use a Python list

2. Balanced Parentheses: The Stack Problem (concepts: pyBalancedBrackets, pyStackParsing)

   This is the single most common beginner stack problem. It shows up in FAANG phone screens, DE interviews, and platform engineering rounds. Given a string of brackets, determine if it is balanced. Balanced means every open bracket has a corresponding close bracket in the correct order. The stack solution is elegant: push open brackets, pop and validate on close brackets. If the stack is empty at the end, it is balanced. Step Through It Carefully Walk through '([{}])' character by character. Push

3. Queue Fundamentals: The FIFO Discipline (concepts: pyQueueFundamentals, pyFIFO, pyBFS)

   A queue is a line. First in, first out. The person who arrived first gets served first. In Python, the right tool is collections.deque, not a list. Here is why this matters in an interview: list.pop(0) is O(n) because Python has to shift every element left. deque.popleft() is O(1) because it is a doubly-linked list under the hood. If you use list as a queue in a coding interview at Google, the interviewer will catch it. Use deque. Every time. FIFO Intuition and Why It Matters FIFO guarantees fai

4. Monotonic Stack: The Next Greater Element (concepts: pyMonotonicStack, pyNextGreater)

   The next greater element problem is the gateway to monotonic stacks. Given an array, for each element, find the next element to its right that is larger. Brute force: for each element, scan right until you find something larger. O(n squared). Monotonic stack: one pass, O(n). The key insight: use the stack to track elements that have not yet found their 'next greater' element. When a large element arrives, it resolves all smaller elements waiting in the stack. Next Greater Element — Step by Step

5. Common Bugs and Python Pitfalls (concepts: pyStackBugs, pyQueuePerf, pyRecursionLimit)

   These are the bugs that experienced engineers make in interviews when they are moving fast. Every one of them is preventable if you know to look for it. Read this section and burn it into your muscle memory before your interview. Bug 1: list.pop(0) Instead of deque.popleft() This is the most common Python-specific mistake. list.pop(0) works correctly but is O(n). In a BFS over a graph with V vertices and E edges, using list.pop(0) makes BFS O(V * (V + E)) instead of O(V + E). On a graph with 10,