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2. Valid Parentheses

easyAsked at Instacart

Validate that brackets close in the correct order — Instacart uses this as a stack-discipline check before more involved order-state validation problems.

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

Problem

Given a string containing only '()[]{}', determine if the input 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 only of '()[]{}'

Examples

Example 1

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

Example 2

Input
s = "(]"
Output
false

Approaches

1. Brute force replace

Repeatedly strip empty pairs until nothing changes.

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, pop and match closers against the top of the stack.

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

Tradeoff:

Instacart-specific tips

Instacart often pivots this into 'validate a shopper bagging sequence' — be ready to extend the stack to handle multi-bag aisle constraints.

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Output

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