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2. Valid Parentheses

easyAsked at Spotify

Determine if a string of brackets is balanced with proper nesting.

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

Problem

Given a string containing just the characters '()[]{}', determine if the input string is valid. Brackets must close in the correct order and every open 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

Repeatedly remove matched pairs until none remain

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, pop and match closers against a map. Reject early on mismatch.

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

Tradeoff:

Spotify-specific tips

Spotify often pairs this with audio-pipeline parsing follow-ups (config files, query syntax), so mention stacks as a building block before they ask.

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