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2. Valid Parentheses

easyAsked at Monzo

Validate that a string of brackets is properly nested and closed.

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. An input string is valid if open brackets are closed by the same type of brackets and in the correct order.

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. Naive scan

Scan and try to match without state.

Time
O(n^2)
Space
O(1)
// Naive cannot track nesting; will fail on "([)]".

Tradeoff:

2. Stack

Push openers; for closers verify the top matches.

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

Tradeoff:

Monzo-specific tips

Monzo expects you to think about malformed input — apply the same defensive instincts you would for ledger entries.

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Output

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