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2. Valid Parentheses

easyAsked at Postman

Given a string of brackets, determine if every opening bracket has a matching closing bracket in the correct order.

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. Brackets must close in the correct order and every closer must match the same type of opener.

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 inner pairs until the string is empty or unchanged.

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 openers onto a stack; on each closer pop and verify it matches.

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

Tradeoff:

Postman-specific tips

Postman uses this to gauge how candidates validate nested JSON request bodies or matched braces in collection variables.

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Output

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