Overview

Given a string containing brackets (), [], {} and other characters, determine whether every opening bracket has a corresponding closing bracket in the correct order.

Implement isBalanced(str) that returns true if all brackets are properly nested and matched, and false otherwise.

Constraints

• Input is always a string (may be empty).
• Only (), [], and {} are considered brackets — all other characters should be ignored.
• An empty string is considered balanced.
• Brackets must be closed in the correct order: "([])" is valid, "([)]" is not.
• Strings with only non-bracket characters are considered balanced.

Examples

isBalanced("()[]{}");           // true
isBalanced("([{}])");           // true
isBalanced("(]");               // false
isBalanced("([)]");             // false
isBalanced("a]b(c)[d]");       // false — stray closing bracket
isBalanced("");                 // true
isBalanced("hello world");     // true — no brackets

Notes

• A classic stack-based approach works well here — push opening brackets, pop on closing brackets, and verify the match.
• Edge cases: strings with only opening brackets, only closing brackets, deeply nested brackets, and mixed non-bracket characters.
• Time complexity should be $O(n)$ where $n$ is the string length.

Solution

function isBalanced(str) {
  const stack = [];
  const pairs = { ")": "(", "]": "[", "}": "{" };
  const openers = new Set(["(", "[", "{"]);

  for (const ch of str) {
    if (openers.has(ch)) {
      stack.push(ch);
    } else if (pairs[ch] !== undefined) {
      if (stack.length === 0 || stack.pop() !== pairs[ch]) return false;
    }
  }

  return stack.length === 0;
}

Resources

• Stack Data Structure — Wikipedia
• Matching Brackets — LeetCode