0x60Lesson 7 of 13

Match and order with stacks and queues

Use LIFO and FIFO structures for nesting, undo, and level-by-level work.

20 min 5-question quiz 1 code exercise

By the end of this lesson you can

Choose between a stack and a queue.
Validate nested brackets with a stack.
Use a deque for efficient queue operations.

A stack is last-in, first-out (LIFO): push and pop at the same end. It fits anything nested or reversible - matching brackets, undo history, evaluating expressions, depth-first search. A queue is first-in, first-out (FIFO): add at the back, remove at the front. It fits processing in arrival order and breadth-first search.

Stacks and queues in Python

Stack: a plain list - append() pushes and pop() pops, both O(1).
Queue: collections.deque - append() adds and popleft() removes, both O(1). Avoid list.pop(0), which is O(n).
Monotonic stack: a stack kept in increasing or decreasing order finds the “next greater element” for every item in O(n) total.

solution.py

1def is_balanced(text):
2    pairs = {")": "(", "]": "[", "}": "{"}
3    stack = []
4    for char in text:
5        if char in "([{":
6            stack.append(char)
7        elif char in pairs:
8            if not stack or stack.pop() != pairs[char]:
9                return False
10    return not stack
11
12print(is_balanced("{[()()]}"), is_balanced("([)]"), is_balanced("(("))

Output

True False False

Every opening bracket waits on the stack for its partner; the most recent unmatched opener must close first, which is exactly LIFO order. Two edge cases catch many candidates: a closing bracket when the stack is empty, and openers left on the stack at the end.

Key takeaways

Stacks are LIFO: nesting, undo, DFS, monotonic-stack problems.
Queues are FIFO: arrival order, BFS.
Use deque for queues in Python.

Lesson quiz

5 questions · pass with 4 correct · up to 50 XP

Passing this quiz completes the lesson and keeps your streak going. Questions you miss come back in review sessions later.

Practice: solve interview problems

Solve a classic interview problem in Python and run it against test cases. Aim for the optimal complexity, then check the edge cases. Exercises run locally in your browser.

Exercise 1

Validate brackets

+25 XP

Read a string made of the characters ()[]{}. Print valid if every bracket is closed by the same type in the correct order, otherwise invalid.

Nested
Crossed
Unclosed
Starts with a closer

main.py

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