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2. Valid Parentheses

easyAsked at Chegg

Validate balanced brackets using a stack — Chegg uses this to check stack fluency for parsing student LaTeX answers.

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

Problem

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

Examples

Example 1

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

Example 2

Input
s = "(]"
Output
false

Approaches

1. Repeated replace

Strip empty pairs until no change.

Time
O(n^2)
Space
O(n)
let prev;
do { prev = s; s = s.replace(/\(\)|\[\]|\{\}/g, ''); } while (s !== prev);
return s === '';

Tradeoff:

2. Stack

Push openers; on closer verify it matches the top of the stack. Empty stack at end means valid.

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

Tradeoff:

Chegg-specific tips

Chegg likes candidates who frame this as a parser warm-up because their student-answer ingestion pipeline tokenizes LaTeX and code submissions with stack-based validators.

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Output

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