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2. Valid Parentheses

easyAsked at Brex

Determine if a string of brackets is properly opened and closed in the correct order.

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

Problem

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', 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 replace

Repeatedly remove '()','[]','{}' until string stops changing.

Time
O(n^2)
Space
O(n)
while (s.includes('()')||s.includes('[]')||s.includes('{}'))
  s = s.replace('()','').replace('[]','').replace('{}','');
return s==='';

Tradeoff:

2. Stack

Push opens, pop and check on closes. Empty stack at end means valid.

Time
O(n)
Space
O(n)
function isValid(s) {
  const stack = [];
  const pairs = { ')': '(', ']': '[', '}': '{' };
  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:

Brex-specific tips

Brex frames bracket-matching as a check for well-formed approval-chain expressions, so name the analogy aloud and discuss what to do when the stream is malformed at byte 50,000.

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Output

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