Feb. 17, 2018 - 2:16 PM, continue to implement and test code written in mock interview. The design has several bugs by adding left open parentheses.

MinimumRequiredParentheses_afterMockInterview.cs

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

namespace FindMinimumParentheses
{
    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)
        {
          if(text == null || text.Length == 0)   
              return "";
  
          var length = text.Length; 
          int openBracket = 0;
          
          var minimumString = new StringBuilder(); 
    
          char previous = ' ';
          for(int index = 0; index < length; index++)
          {
              var current = text[index]; 
              var isOpen = current == '('; 
      
              if(isOpen)
              {
                  openBracket++; 
                  // add (
                  if(openBracket == 1)
                  {
                      minimumString.Append("("); 
                  }
              }
              else 
              {
                  if(openBracket > 0)
                  {
                      // add - previous char ), I have to special handling , add ()
                      // missing open bracket somewhere before 
                      if(previous == '(')
                      {
                          minimumString.Append("()"); // 
                      }
                      else 
                      {  // 1. (()) -> no need to add - extra )
                         // 2. ()()) -> need to add extra big parenthese ->  
                          minimumString.Append(")"); // but I missed one open bracket at the beginng
                      }
              
                      openBracket--; 
                  }         
              }
      
               previous = current; 
             }    
           
            // edge case
            var validString = minimumString.ToString(); 
            if(openBracket > 0)
            {
                var openBracketIndex = getExtraOpenBracketIndex(minimumString.ToString());                 
                if(openBracketIndex >= 0)
                {
                    return openBracketIndex == 0 ? validString.Substring(1) : 
                        (validString.Substring(0, openBracket) + 
                        validString.Substring(openBracket + 1, validString.Length - openBracket - 1));
                }
            }
    
            return minimumString.ToString(); 
        }

        private static int getExtraOpenBracketIndex(string s)
        {
            if(s == null || s.Length == 0)
            {
                return -1;
            }          

            // remove the last one open bracket 
            var length = s.Length; 
            
            for (int i = 0; i < length; i++)
            {
                var current = s[length - 1 - i];
               
                if (current == '(')
                {
                    return i; 
                }
            }

            return -1; 
        }
    }
}