Minimum substring solution, pass all test cases.

minimumSubstring_passAllTestcases.cs

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

namespace SmallestSubstringProject
{
    class Program
    {
        static void Main(string[] args)
        {   // ["A","B","C"], "ADOBECODEBANCDDD"
            var minimumSubstring = GetShortestUniqueSubstring(new char[] { 'A', 'B', 'C' }, "ADOBECODEBANCDDD"); 
        }

        /// <summary>
        /// code review on Dec. 2, 2017
        /// </summary>
        /// <param name="source"></param>
        /// <param name="search"></param>
        /// <returns></returns>
        public static string GetShortestUniqueSubstring(char[] source, string search) // "xyyzyzyx", ['x','y','z']
        {
            if(source == null || source.Length == 0 || search.Length == 0) // false
            {
                return string.Empty; 
            }

            var length = source.Length;// 3
            var searchLength = search.Length; // 8
            
            var minimumSubstring = "";
            var minimumLength = searchLength + 1;// 9 
            var uniqueCharsMet = 0; 

            int left = 0;

            var dictionary = source.ToDictionary(c=>c, c=>1); // dictionary['x'] = 1, same as 'y', 'z'

            for (int index = 0; index < searchLength; index++) // 0, 1, 2, 3, 4
            {
                var visit = search[index]; // 'x', 'y', 'y', 'z', 'y'
                var isInSet = dictionary.ContainsKey(visit); // true, true, true, true, true

                if(!isInSet)
                {
                    continue; 
                }

                var isNeeded = dictionary[visit] > 0; // true, true, false, true, false
                if(isNeeded)
                {
                    uniqueCharsMet++; // 1, 2, 3
                }

                dictionary[visit]--; // d['x'] = 0; d['y'] = 0; d['y'] = -1, d['z'] = 0, d['y'] = -2

                while(uniqueCharsMet == length) // 1 != 3, false; 2 != 3, false; 2!= 3, false; true, false
                {
                    // check to see if the left point can move forward
                    if( !dictionary.ContainsKey(search[left]) || dictionary[search[left]] <= 0 ) // "xyyz"
                    {
                        var current = search[left]; // 'x'                         

                        if (dictionary.ContainsKey(current) && dictionary[current] <= 0) // true
                        {
                            var count = dictionary[current];
                            
                            if (count == 0) 
                            {
                                uniqueCharsMet--; // 2
                            }
                            
                            dictionary[current]++; // dictionary['x'] = 1                            
                        }

                        // maintain substring length
                        var startPosition = left; // 0 
                        var currentLength = index - startPosition + 1; // 4
                        var currentSubstring = search.Substring(startPosition, currentLength); //"xyyz"
                        if(currentLength < minimumLength) // 4 < 9
                        {
                            minimumLength = currentLength; // 4
                            minimumSubstring = currentSubstring; // "xyyz"
                        }

                        left++; // 1
                    }
                }
            }

            return minimumLength == (searchLength + 1) ? string.Empty : minimumSubstring; 
        }
    }
}