LeetCode-93.Restore IP Addresses

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

For example:
Given "25525511135",

return ["255.255.11.135", "255.255.111.35"]. (Order does not matter)

public class Solution 
{
    public IList<string> RestoreIpAddresses(string s)
    {
        IList<string> list = new List<string>();
        Fun(list, "",0, s);
        return list;
    }
    private void Fun(IList<string> list,string item, int dot, string s)
    {
         if (s.Length > (4 - dot) * 3)
            return;
            
        if (dot==3 && IsValid(s))
        {
            item += s;
            list.Add(item);
            return;
        }

        for (int i = 1; i <= 3; i++)
        {
            if (s.Length>i)
            {
                string temp = s.Substring(0, i);
                if (IsValid(temp))
                {
                    Fun(list, item + temp + ".", dot + 1, s.Substring(i));
                } 
            }
        }
    }

    private bool IsValid(string v)
    {
        if (v.StartsWith("0")&&v.Length>1)
            return false;
        int n = Convert.ToInt16(v);
        return 0 <= n && n <= 255;
    }
}

public class Solution 
{
    public IList<string> RestoreIpAddresses(string s)
    {
        IList<string> list = new List<string>();
        int len = s.Length;
        for (int i = 1; i < 4; i++)
        {
            for (int j = 1; j < 4; j++)
            {
                for (int k = 1; k < 4; k++)
                {
                    if (i + j + k + 3 >= len&& i + j + k < len)
                    {
                        string s1 = s.Substring(0, i), s2 = s.Substring(i, j), s3 = s.Substring(i + j, k), s4 = s.Substring(i + j + k);
                        if (IsValid(s1) && IsValid(s2) && IsValid(s3) && IsValid(s4))
                        {
                            list.Add(s1 + "." + s2 + "." + s3 + "." + s4);
                        }
                    }
                }
            }
        }
        return list;
    }
    
    private bool IsValid(string v)
    {
        if (v.StartsWith("0") && v.Length > 1)
            return false;
        int n = Convert.ToInt16(v);
        return 0 <= n && n <= 255;
    }
}