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2. Valid Parentheses

easyAsked at Duolingo

Determine if an input string of brackets is correctly opened and closed.

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

Problem

Given a string containing only the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. Brackets must close in the correct order and every opening bracket has a matching closer of the same type.

Constraints

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

Examples

Example 1

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

Example 2

Input
s = "(]"
Output
false

Approaches

1. Brute force replace

Repeatedly remove '()', '[]', '{}' until no change; valid iff string is empty.

Time
O(n^2)
Space
O(n)
let prev;
do { prev = s; s = s.replace(/\(\)|\[\]|\{\}/g, ''); } while (s !== prev);
return s.length === 0;

Tradeoff:

2. Stack

Push each opener; on a closer pop and verify it matches. Empty stack at end means balanced.

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

Tradeoff:

Duolingo-specific tips

Duolingo lesson-tree validation runs on similar nested structures, so highlight how a stack maps cleanly to a balanced-skill-DAG check.

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