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2. Valid Parentheses

easyAsked at Nubank

Validate that nested bracket structures in a KYC document parser are balanced.

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. Brackets must close in the correct order and every open bracket needs a matching close.

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. Brute force

Repeatedly remove matched pairs until none remain.

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; on closer, pop and compare. Empty stack at end means valid.

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

Tradeoff:

Nubank-specific tips

Nubank uses parser-style problems to probe whether you can reason about malformed third-party KYC payloads without crashing the ingestion pipeline.

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Output

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