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2. Valid Parentheses

easyAsked at Klarna

Determine if a string of brackets is balanced and properly nested.

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

Problem

Given a string s containing only the characters '()[]{}', determine if the input string is valid. A string is valid if open brackets are closed by the same type and 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. Brute force replace

Repeatedly remove matched pairs.

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 a 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:

Klarna-specific tips

Klarna engineers care about stack discipline since their installment plan parsers and risk-rule DSLs depend on balanced expression validation.

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