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2. Valid Parentheses

easyAsked at Yelp

Verify a string of brackets is well-formed using a stack — Yelp uses this to test the same nesting validation it runs on user-submitted query strings for advanced search filters.

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

Problem

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

Repeatedly strip empty pairs until none left.

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 pairs = { ')': '(', ']': '[', '}': '{' };
  const stack = [];
  for (const c of s) {
    if (!(c in pairs)) stack.push(c);
    else if (stack.pop() !== pairs[c]) return false;
  }
  return stack.length === 0;
}

Tradeoff:

Yelp-specific tips

Yelp grades cleanliness of the stack-popping loop and will follow up by asking how you'd validate a deeply nested business-filter query string before passing it to their search service.

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Output

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