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2. Valid Parentheses

easyAsked at Grab

Validate matched brackets using a stack — a Grab classic for testing data-structure intuition.

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 each opening bracket must have 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 replace

Repeatedly remove matched pairs until nothing changes.

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, pop on close and compare. Mismatches or leftover opens mean invalid.

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

Tradeoff:

Grab-specific tips

Grab interviewers like candidates who frame this as validating nested JSON in a GrabPay receipt — concrete framing earns bonus signal.

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Output

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