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CODING CHALLENGE · N°02

Valid Parentheses

Easy StackStrings

The canonical stack problem: decide whether every bracket is closed by the right type, in the right order. A stack turns nested matching into a single pass. Solve it in Python or TypeScript.

The problem

Given a string s of just the characters ()[]{}, decide whether it is valid: every open bracket is closed by a bracket of the same type, and brackets close in the right order (the most recently opened closes first). Return a boolean.

EXAMPLE 1
Input s = "()"
Output true
EXAMPLE 2
Input s = "()[]{}"
Output true
EXAMPLE 3
Input s = "(]"
Output false
wrong type
EXAMPLE 4
Input s = "([)]"
Output false
wrong order
EXAMPLE 5
Input s = "{[]}"
Output true
properly nested
CONSTRAINTS
  • 1 ≤ s.length ≤ 10⁴
  • s contains only the six bracket characters.
  • A stack gives O(n) time and O(n) space.
SOLVE IT YOURSELF

Your turn — write it

Edit the stub, hit Run (or ⌘/Ctrl + Enter), and watch the hidden tests. Stuck? the hints are right above and Reveal solution is one click away.

YOUR TASK

Implement is_valid(s): return True only if the brackets are balanced and correctly nested. Push opens onto a stack; on a close, the top of the stack must be its matching open.

HINTS — 4 IDEAS
  1. The most recently opened bracket must be the first one closed — that is exactly a stack (last in, first out).
  2. Push every opening bracket. On a closing bracket, pop and check it matches.
  3. A map from closing → opening makes the match check one lookup.
  4. It is invalid if a close finds an empty stack (nothing to match) — and at the end the stack must be empty.
CPython · WebAssembly

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