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2. Valid Parentheses

easyAsked at Slack

Given a string of brackets, decide whether they are balanced.

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

Problem

Given a string s containing the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input 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 only of brackets

Examples

Example 1

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

Example 2

Input
s = "(]"
Output
false

Approaches

1. Brute force

Repeatedly remove matched pairs until stable.

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 opens, pop and verify on closes. Empty stack at the end means balanced.

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

Tradeoff:

Slack-specific tips

Slack reuses bracket-matching logic in message formatting (mrkdwn) — expect a follow-up about parsing Slack's *bold* / `code` / >quote markers.

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Output

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