Minimum Window Substring in C#

MinWindow.cs

var s = "ADOBZASQWECCCODEBANCNMKLNJNCBA";
var substring = "ABC";

Console.WriteLine(MinWindow(s, substring));

string MinWindow(string s, string substring)
{
    var subsDictionary = new Dictionary<char, int>();
    var window = new Dictionary<char, int>();

    var matches = 0;
    var need = substring.Length;

    var currentSolutionStart = 0;
    var solutionLength = 0;

    var left = 0;

    foreach (var letter in substring)
    {
        if (subsDictionary.ContainsKey(letter))
        {
            subsDictionary[letter] += 1;
        }
        else
        {
            subsDictionary.Add(letter, 1);
            window.Add(letter, 0);
        }
    }

    for (int right = 0; right < s.Length; right++)
    {
        var c = s[right];
        if (window.ContainsKey(c))
        {
            window[c] += 1;

            if (window[c] == subsDictionary[c])
            {
                matches++;
            }

            while (matches == need)
            {
                if (right - left + 1 < solutionLength || solutionLength == 0)
                {
                    solutionLength = right - left + 1;
                    currentSolutionStart = left;
                }

                if (window.ContainsKey(s[left]))
                {
                    window[s[left]] -= 1;
                    if (window[s[left]] < subsDictionary[s[left]])
                    {
                        matches--;
                    }
                }
                left++;
            }
        }
    }

    if (solutionLength == 0)
    {
        return "";
    }

    return s.Substring(currentSolutionStart, solutionLength);
}


//credits: https://www.youtube.com/watch?v=jSto0O4AJbM