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2. Valid Parentheses

easyAsked at GitLab

Validate that a string of brackets is properly nested and closed in order.

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

Problem

Given a string s containing only the characters '()[]{}', return true if every open bracket is closed by the matching type in the correct order, false otherwise.

Constraints

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

Examples

Example 1

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

Example 2

Input
s="(]"
Output
false

Approaches

1. Brute force replace

Repeatedly strip empty pairs until no change.

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

Tradeoff:

2. Stack

Push opens, pop on close and verify match.

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

Tradeoff:

GitLab-specific tips

GitLab interviewers often follow up by asking how this would handle CI-pipeline YAML or markdown nesting — keep an extensible mapping in mind.

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