撒列实现关键字过虑

public string Filter(string str)
        {
            if (string.IsNullOrEmpty(str))
            {
                return string.Empty;
            }
            int len = str.Length - 1;
            char[] sb = str.ToCharArray();
            bool isOK = true;
            for (int i = 0; i <= len; i++)
            {
                if (keyDict.ContainsKey(str[i]))
                {
                    int j = i;
                    foreach (string s in keyDict[str[i]])
                    {
                        foreach (char c in s)
                        {
                            if ( j >= len || c != str[j++])
                            {
                                isOK = false;
                                break;
                            } 
                        }
                        if (isOK)
                        {
                            for (int k = i; k < j; k++)
                                sb[k] = '*';
                            i = j;
                            break;
                        }
                        else
                        {
                            j=i;
                            isOK = true;
                        }
                    } 
                } 
            }
            return new string(sb);
        }