Data Structures - lesson 3 | HackIndia Skills

Data Structures - lesson 3 | HackIndia Skills | HackIndia

Example

Stack example

Put this in your repo as code/python/valid_parentheses.py. It shows the real stack pattern and has no extra classes.

def is_valid(s: str) -> bool:
    pairs = {')': '(', ']': '[', '}': '{'}
    stack = []

    for ch in s:
        if ch in pairs.values():
            stack.append(ch)
        elif ch in pairs:
            if not stack or stack.pop() != pairs[ch]:
                return False
        else:
            return False

    return len(stack) == 0


def test_is_valid():
    assert is_valid("()[]{}") is True
    assert is_valid("([{}])") is True
    assert is_valid("(]") is False
    assert is_valid("((") is False

Example

Queue example

For queues, prefer deque in Python. This small BFS shape will repeat later when you reach trees and graphs.

from collections import deque

def time_to_process(tasks):
    q = deque(tasks)
    order = []

    while q:
        order.append(q.popleft())

    return order


def test_time_to_process():
    assert time_to_process(["A", "B", "C"]) == ["A", "B", "C"]

Tip

Write comparison notes

Your kit should not just dump solutions. Add short notes that explain why this structure fits.

A good note says: access pattern, main operations, costs, and why you rejected another option.

# notes/stacks-queues-linked-lists.md

Problem: Valid parentheses
Pick: Stack
Why: Need to match the latest unmatched opening bracket first.
Cost: O(n) time, O(n) space in the worst case.
Rejected: Queue, because oldest bracket first does not match nesting.

Problem: Process tasks in arrival order
Pick: Queue using deque
Why: First task should be processed first.
Cost: O(n) time, O(n) space.
Rejected: Python list with pop(0), because removing from the front shifts items.

Problem: Detect linked list cycle
Pick: Slow and fast pointers
Why: We only move through next links. No index access is needed.
Cost: O(n) time, O(1) space.
Rejected: Array copy, because it uses extra space and changes the problem shape.

Stacks queues linked lists

One idea

When to use stack

When to use queue

Linked list trap

Stack example

Queue example

Write comparison notes

Do this now

Quick check