Reverse Substrings Between Each Pair of Parentheses

export function reverseParentheses(s: string): string {
  const stack = [''];
  for (const char of s) {
    if (char === '(') {
      stack.push('');
    } else if (char === ')') {
      const prev = stack.pop()!;
      stack[stack.length - 1] += reverseString(prev);
    } else {
      stack[stack.length - 1] += char;
    }
  }
 
  return stack.join('');
}
 
function reverseString(s: string) {
  return [...s].reverse().join('');
}