Problem

Given a string of parentheses, write a function to determine whether the string is balanced. A string is balanced if there are an equal number of open and closed parentheses, and each open parentheses is matched with a closed parentheses.
Example input: "((()))"
Example output: true
Example input: "()())"
Example output: false

Solution

function isBalanced(str) {
  let open = 0;
  let closed = 0;
  for (let i = 0; i < str.length; i++) {
    if (str[i] === "(") {
      open++;
    } else if (str[i] === ")") {
      closed++;
    }
  }
  return open === closed;
}

// This solution is optimal because it only iterates through the string once, and it does not use any extra space.