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2. Valid Parentheses

easyAsked at MercadoLibre

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

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

Problem

Given a string s containing only the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input is valid if open brackets are closed by the same type in the correct order.

Constraints

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

Examples

Example 1

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

Example 2

Input
s = "(]"
Output
false

Approaches

1. Brute force

Repeatedly strip 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 opens; on a close, pop and verify pair. Stack must be empty at the end.

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

Tradeoff:

MercadoLibre-specific tips

MercadoLibre uses this to filter for stack intuition — handy for nested cart/checkout flows where each step opens state that must close cleanly.

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Output

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