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2. Valid Parentheses

easyAsked at Glassdoor

Validate a string of brackets using a stack — Glassdoor uses this as a warm-up to gauge basic data-structure fluency.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if open brackets are closed by the same type of brackets in the correct order.

Constraints

  • 1 <= s.length <= 10^4
  • s consists of parentheses only

Examples

Example 1

Input
s = "()[]{}"
Output
true

Example 2

Input
s = "(]"
Output
false

Approaches

1. Brute force replace

Repeatedly remove '()', '[]', '{}' until none remain.

Time
O(n^2)
Space
O(n)
while (s.includes('()') || s.includes('[]') || s.includes('{}')) {
  s = s.replace('()','').replace('[]','').replace('{}','');
}
return s.length === 0;

Tradeoff:

2. Stack

Push openers; pop and match on closers.

Time
O(n)
Space
O(n)
function isValid(s) {
  const map = { ')':'(', ']':'[', '}':'{' };
  const stack = [];
  for (const c of s) {
    if (!map[c]) stack.push(c);
    else if (stack.pop() !== map[c]) return false;
  }
  return stack.length === 0;
}

Tradeoff:

Glassdoor-specific tips

Glassdoor interviewers like clean stack code with a one-line bracket map — they parse millions of structured review payloads, so they grade for tidy state-machine instincts.

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Output

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