020. Valid Parentheses

At a Glance

• Topic: Stack
• Pattern: Stack
• Difficulty: Easy
• LeetCode: 020

Problem Statement

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

Open brackets must be closed by the same type of brackets.
Open brackets must be closed in the correct order.
Every close bracket has a corresponding open bracket of the same type.

Example 1:

Input: s = "()"

Output: true

Example 2:

Input: s = "()[]{}"

Output: true

Example 3:

Input: s = "(]"

Output: false

Example 4:

Input: s = "([])"

Output: true

Example 5:

Input: s = "([)]"

Output: false

Constraints:

1 <= s.length <= 104
s consists of parentheses only &#39;()[]{}&#39;.

Approach & Solution Steps

Use a stack to keep track of opening brackets. Iterate through the string. If it's an opening bracket, push to stack. If closing, pop from stack and check if it matches the current closing bracket.

Optimal JS Solution

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

Edge Cases & Pitfalls

• Always consider empty or null inputs.
• Watch out for off-by-one index errors.