Leetcode 76 minimum windows substring - C# mocking practice, timeout on Leetcode online judge. Last test case timeout

Leetcode76_minimumWindowSubstring.cs

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

namespace Leetcode76MinimumWindowSubstring_mocking
{
    class Program
    {
        static void Main(string[] args)
        {
        }

        public static string GetSmallestSubstring(string input, string unique)
        {
          if(input == null || input.Length == 0 || unique == null || unique.Length == 0)
          {
            return string.Empty; 
          }
      
          // assume that input is not empty, unique is not empty
          int minimumLength = Int32.MaxValue; 
          var hashset = new HashSet<char>(); 
      
          foreach(var item in unique)
          {
            hashset.Add(item); 
          }
      
          var dictionary = new Dictionary<char, int>(); 
      
          int left = 0; 
          int length = input.Length; 
          string minimumString = string.Empty; 
      
          for(int i = 0; i < length; i++)
          {
            var visit = input[i]; 
        
            //put into dictionary
            if(dictionary.ContainsKey(visit))
            {
              dictionary[visit] ++; 
            }
            else 
            {
              dictionary.Add(visit, 1); 
            }
        
            // if all unique chars is in the dictionary
            bool isFound = checkFound(hashset, dictionary); 
        
            if(!isFound)   
            {
              continue; 
            }                   
        
            while( left < length && 
                  (!hashset.Contains(input[left])      ||
                  (dictionary.ContainsKey(input[left]) && 
                   dictionary[input[left]] > 1)))
            {
              var current = input[left];
              dictionary[current]--; 
              left ++; 
            }
           
            var currentLength = i - left + 1; 
            if( currentLength < minimumLength)
            {
              minimumLength = currentLength;           
              minimumString = input.Substring(left, currentLength); 
            }
        
            left ++;         
          }

          return minimumString; 
        }

        /// <summary>
        /// code review on June 30, 2017
        /// </summary>
        /// <param name="hashset"></param>
        /// <param name="dictionary"></param>
        /// <returns></returns>
 
        private static bool checkFound(HashSet<char> hashset, Dictionary<char, int> dictionary)
        {
            foreach (var item in hashset)
            {
                if (!dictionary.ContainsKey(item) || dictionary[item] == 0)
                {
                    return false;
                }
            }

            return true;
        }
    }
}