public class Solution
{
    public int LengthOfLongestSubstring(string s)
    {
        var set = new HashSet<char>();
        int left = 0, maxLength = 0;

        for (int right = 0; right < s.Length; right++)
        {
            while (set.Contains(s[right]))
            {
                set.Remove(s[right]);
                left++;
            }
            
            set.Add(s[right]);
            maxLength = Math.Max(maxLength, right - left + 1);
        }
        
        return maxLength;
    }
}

Span<int> number = stackalloc int[5];
for(int i = 0; i < 5; i++)
    number[i] = i;

Console.WriteLine(string.Join("," , number.ToArray()));