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2. Valid Parentheses

easyAsked at Wise

Validate that a string of brackets is balanced — a classic stack warm-up before Wise drills you on ledger entries that must balance debits and credits.

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

Problem

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

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 matching pairs until no change.

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 on closers. Linear and clean.

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

Tradeoff:

Wise-specific tips

Wise interviewers will probe whether you can map this to ledger-pair invariants — every credit needs a matching debit, just like every opener needs a closer.

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Output

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