0093-restore-ip-addresses.cs

public class Solution 
{
	public IList<string> RestoreIpAddresses(string s)
	{
		if (s.Length < 4) return [];

		var output = new List<string>();
		Backtrack(output, new StringBuilder(), s, null, 0, 0);
		return output;
	}
	private void Backtrack(List<string> output, StringBuilder sb, ReadOnlySpan<char> input, int? octet, int octet_count, int index)
	{
		if (octet_count > 4) return;

		if (sb.Length - 3 == input.Length)
		{
			output.Add(sb.ToString());
		}
		else
		{
			int max_index = Math.Min(index + 3, input.Length);
			for (int i = index; i < max_index; i++)
			{
				if (octet.HasValue)
				{
					if (octet.Value == 0 || octet > 25 || octet == 25 && input[i] > '5') break;

					octet *= 10;
					octet += input[i] - '0';
				}
				else
				{
					octet = input[i] - '0';
				}
				string octet_string = sb.Length == 0 ? octet.Value.ToString() : $".{octet.Value}";
				sb.Append(octet_string);
				Backtrack(output, sb, input, null, octet_count + 1, i + 1);
				sb.Remove(sb.Length - octet_string.Length, octet_string.Length);
			}
		}
	}
}