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2. Valid Parentheses

easyAsked at Udemy

Validate that brackets nest correctly — a stack warm-up Udemy uses to gauge whether you reach for the right data structure before the harder DRM/search rounds.

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

Problem

Given a string containing the characters '(){}[]', determine whether the brackets are matched and properly nested. Return true if valid and false otherwise.

Constraints

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

Examples

Example 1

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

Example 2

Input
s = "(]"
Output
false

Approaches

1. Brute force string replace

Repeatedly strip empty pairs until the string is unchanged.

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

Tradeoff:

2. Stack

Push openers; on a closer, the top of the stack must be its match. The string is valid iff the stack ends empty.

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

Tradeoff:

Udemy-specific tips

Frame this as validating nested course curriculum structures (sections inside lessons inside courses) — Udemy interviewers like candidates who connect the stack pattern to their content tree.

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Output

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