ALGORITHM READING NOTE

Valid Parentheses

Valid Parentheses checks whether every opening bracket is closed by the matching bracket in the correct nesting order. A stack models that nesting naturally.

TYPEAlgorithmCATEGORYClassic ProblemsDEPTHBeginnerTYPICAL COMPLEXITYVaries by implementation / input constraints

SHORT INTERVIEW ANSWER

Start with this mental model

Valid Parentheses checks whether every opening bracket is closed by the matching bracket in the correct nesting order. A stack models that nesting naturally.

SIMPLE EXPLANATION

How it works, without unnecessary jargon

Push opening brackets.

For each closing bracket, pop and verify that it matches. At the end the stack must be empty.

EXECUTION

Read the algorithm as a sequence of decisions

Restate constraints and edge cases.

Identify the pattern and invariant.

Walk through a small example before coding.

A small example to keep in mind

Read a tiny input by hand and write down the state after every important operation. For Valid Parentheses, the goal is to make the invariant visible rather than to memorize code.

READABLE JAVASCRIPT IMPLEMENTATIONJavaScript

function isValid(input) {
  const match = { ")": "(", "]": "[", "}": "{" };
  const stack = [];
  for (const ch of input) {
    if ("([{ ".includes(ch) && ch !== " ") stack.push(ch);
    else if (match[ch] && stack.pop() !== match[ch]) return false;
  }
  return stack.length === 0;
}

How to read this code

Do not memorize the syntax. Follow the state that changes on each line and connect it to the invariant described above.

COMPLEXITY

Varies by implementation / input constraints

Do not quote this complexity mechanically. Ask which operation or input dimension it describes, then derive the cost from the loops, recursion depth, data-structure operations and any preprocessing assumptions.

WHEN TO USE IT

Good fit

Use the recognized pattern when its invariant matches the problem constraints.

COMMON MISTAKES

What usually goes wrong

Jumping to code before defining the invariant.
Optimizing before producing a correct baseline.

HOW TO EXPLAIN IT IN AN INTERVIEW

A professional answer shape

This is a classic example of choosing a data structure from the problem's structure: nested last-opened-first-closed behavior directly implies a stack.

Problem→Core idea→Invariant / state→Big-O→Trade-off

QUICK INTERVIEW CHECK

What problem does Valid Parentheses solve?

Valid Parentheses checks whether every opening bracket is closed by the matching bracket in the correct nesting order. A stack models that nesting naturally.

What should I mention about complexity?

Its typical cost here is Varies by implementation / input constraints, but a complete answer should connect that cost to the representation, input constraints and the dominant operations.

What is the most common mistake?

Jumping to code before defining the invariant.

Continue readingProblem patternsEdge casesComplexity trade-offs

function isValid(input) { const match = { ")": "(", "]": "[", "}": "{" }; const stack = []; for (const ch of input) { if ("([{ ".includes(ch) && ch !== " ") stack.push(ch); else if (match[ch] && stack.pop() !== match[ch]) return false; } return stack.length === 0; }