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2. Valid Parentheses

easyAsked at JetBrains

Determine if a string of brackets is balanced — the bread-and-butter check inside every JetBrains parser before AST construction.

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

Problem

Given a string containing only the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. Brackets must close in the correct order and types must match.

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 replacement

Strip empty matched pairs until stable; expensive due to repeated scans.

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

Tradeoff:

2. Stack

Push openers; on a closer, pop and verify match. Single pass linear scan mirrors how parsers track lexer brace depth.

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

Tradeoff:

JetBrains-specific tips

JetBrains expects you to frame this as the same shape as a recursive-descent parser's brace tracker — explicitly map the stack to a parser state.

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Output

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