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2. Valid Parentheses

easyAsked at DigitalOcean

Validate balanced brackets in a string — DigitalOcean uses this to check stack fluency that maps to config-file parsing in Terraform-driven droplet provisioning.

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

Problem

Given a string s containing only the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. Brackets must close in the correct order and be matched by the same type.

Constraints

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

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; on closer, pop and verify matching pair.

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

Tradeoff:

DigitalOcean-specific tips

DigitalOcean engineers want to see you validate input robustly because invalid cloud-init payloads cause silent droplet provisioning failures.

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Output

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