Longest substring with K unique characters

README.txt

This problem was asked by Amazon.

Given an integer k and a string s, 
find the length of the longest substring that 
contains at most k distinct characters.

For example, given s = "abcba" and k = 2, 
the longest substring with k distinct characters is "bcb".

Program.cs

class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(LongestSubstringWithKUniqueChars("abcba", 2)); 
        }

        private static string LongestSubstringWithKUniqueChars(string s, int k)
        {
            if (k == 0)
            {
                return string.Empty;
            }
            Dictionary<char, int> char_count = new Dictionary<char, int>();
            int left = 0, maxL = 0, right = -1, maxR = -1;
            while (right != s.Length - 1)
            {
                if (char_count.ContainsKey(s[right + 1]) || char_count.Count < k)
                {
                    right++;
                    int temp = 1;
                    if (char_count.ContainsKey(s[right]))
                    {
                        temp += char_count[s[right]];
                    }
                    char_count[s[right]] = temp;
                }
                else
                {
                    char_count[s[left]] = char_count[s[left]] - 1;
                    if (char_count[s[left]] == 0)
                    {
                        char_count.Remove(s[left]);
                    }
                    left++;
                }
                if (right - left > maxR - maxL)
                {
                    maxR = right;
                    maxL = left;
                }
            }
            return s[maxL..++maxR];
        }
    }