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2. Valid Parentheses

easyAsked at LINE

Validate a string of brackets — LINE uses it to check whether you reach for a stack instantly when a chat message body has nested formatting.

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

Problem

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

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. Replace pairs in a loop

Keep collapsing '()', '[]', '{}' until the string is empty or no change happens.

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

Tradeoff:

2. Stack

Push openers, pop and compare on closers, and require an empty stack at the end.

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

Tradeoff:

LINE-specific tips

LINE interviewers like when you tie this to validating user-submitted rich-text markup in a chat bubble before render — show product instinct, not just leetcode.

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Output

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