Computer Science Roadmap

Explanation

A stack is a Last-In-First-Out (LIFO) structure. You can think of it as a stack of plates: the last plate you place on top is the first one you take off. The two core operations are push (add an item) and pop (remove the most recently added item). A queue is the opposite order, First-In-First-Out (FIFO): like a line at a shop, the first person to arrive is the first to be served. Queues support enqueue (add to the back) and dequeue (remove from the front). Stacks show up everywhere in computing. Your program call stack tracks which function is running and where to return to. The undo feature in an editor is a stack of past actions, and matching brackets in code is a classic stack problem. Queues appear wherever work is processed in arrival order: a print queue, an event loop handling requests, or a breadth-first search visiting nodes. In Python you can use a list as a stack, pushing with append and popping with pop. For a queue, the standard list is inefficient because removing from the front shifts every element; the collections.deque is designed for this and gives fast operations at both ends. The key idea to internalize is the ordering guarantee, not the implementation. A stack always returns the most recent item; a queue always returns the oldest. Choosing the wrong one changes the whole behavior of an algorithm.

LIFO vs FIFO

How a Stack Works

1push A -> push B -> push C2 3     [C]  <- top4     [B]5     [A]6 7pop() -> C removed, top is now B

How a Queue Works

1enqueue A, B, C2 3  front -> [A][B][C] <- rear4 5  dequeue() -> A removed from the front

Real-World Applications

Syntax

1# Stack (LIFO) using a list2stack = []3stack.append("a")   # push4stack.append("b")   # push5top = stack.pop()   # removes "b"6 7# Queue (FIFO) using deque8from collections import deque9queue = deque()10queue.append("a")   # enqueue11queue.append("b")   # enqueue12front = queue.popleft()  # removes "a"

For a stack use append and pop (both operate on the end). For a queue use append and popleft from collections.deque.

Examples

1stack = []2stack.append(1)3stack.append(2)4stack.append(3)5 6print(stack.pop())7print(stack.pop())8print(stack.pop())

3
2
1

1from collections import deque2 3queue = deque()4queue.append("first")5queue.append("second")6queue.append("third")7 8print(queue.popleft())9print(queue.popleft())10print(queue.popleft())

first
second
third

1def is_balanced(text):2    pairs = {")": "(", "]": "[", "}": "{"}3    stack = []4    for ch in text:5        if ch in "([{":6            stack.append(ch)7        elif ch in ")]}":8            if not stack or stack.pop() != pairs[ch]:9                return False10    return not stack11 12print(is_balanced("(a[b]{c})"))13print(is_balanced("(a[b{c})"))

True
False