Write step 1 algorithm - remove invalid parentheses - simplify the problem, break into two steps, first remove unmatched close bracket first

RemoveInvalidParentheses.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace FindMinimumParentheses
{
    /// <summary>
    /// Step 1 algorithm of mock interview
    /// remove invalid brackets from the input string
    /// - take two steps, first remove unmatched close bracket
    /// - reverse the string, and then redefine the open bracket as ')', 
    ///   remove unmatched close bracket 
    /// - reverse the string back to normal order
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
            var result = GetMinimumRequireParentheses("(())"); // (()) - correct

            var resutl2 = GetMinimumRequireParentheses(")))()"); // (() - wrong

            var result3 = GetMinimumRequireParentheses("(())"); // ()) - wrong 

            var result4 = GetMinimumRequireParentheses("((())"); // ()) - wrong 

            var result5 = GetMinimumRequireParentheses("((()))"); // (())) - wrong 
        }

        public static string GetMinimumRequireParentheses(string text)
        {
            var leftToRight = RemoveUnmatchedCloseBracket(text, '(');
            var reversed    = reverseString(leftToRight);

            var second      = RemoveUnmatchedCloseBracket(reversed, ')');
            return reverseString(second);
        }

        private static string reverseString(string s)
        {
            var array = s.ToCharArray();
            Array.Reverse(array);

            return new string(array); 
        }

        public static string RemoveUnmatchedCloseBracket(string text, char openSign)
        {
          if(text == null || text.Length == 0)   
              return "";
  
          var length = text.Length; 
          int openBracket = 0;
          
          var removedUnmatchedClosed = new StringBuilder();     
         
          for(int index = 0; index < length; index++)
          {
              var current = text[index];
              var isOpen = current == openSign; 
      
              if(isOpen)
              {
                  openBracket++;
                  removedUnmatchedClosed.Append(current); 
              }
              else 
              {
                  if(openBracket > 0)
                  {                                   
                      openBracket--;
                      removedUnmatchedClosed.Append(current);
                  }         
              }                   
          }

          return removedUnmatchedClosed.ToString(); 
        }        
    }
}