Recursive practice - flatten dictionary - Issue is related to time complexity and space complexity analysis

RecursivePractice_flattenDictionary.cs

using System;
using System.Collections.Generic;

class Solution
{
  public static Dictionary<string, string> FlattenDictionary(Dictionary<string, object> dict)
  {
    if(dict == null || dict.Count == 0)
    {
      return new Dictionary<string, string>(); 
    }
    
    var flatten = new Dictionary<string, string>(); 
    
    FlattenDictionaryHelper(dict, "", flatten); 
    
    return flatten; 
  }
  
  /// recursive function
  private static void FlattenDictionaryHelper(Dictionary<string, object> dict, string prefix, Dictionary<string, string> flatten )
  {
    foreach(var pair in dict) // ("Key1", "1") "Key2", Dictionary
    {
      string key   = pair.Key; // "Key1", "Key2"
      Object value = pair.Value; //"1" // dictionary
      
      // https://stackoverflow.com/questions/16956903/determine-if-type-is-dictionary
      string newPrefix = (prefix.Length == 0)? key : ((key.Length == 0)? prefix : (prefix + "." + key)); // "Key1"
      
      var isDictionary = value.GetType() == typeof(Dictionary<string, object>); 
      
      if(!isDictionary)
      {
        
        flatten.Add(newPrefix, (string)value); // ("Key1", "1")
      }
      else 
      {                
        FlattenDictionaryHelper((Dictionary<string, object>)value, newPrefix, flatten); // 
      }
    }
  }

  static void Main(string[] args)
  {

  }
}

// "Key1" -> key, value type -> string 1, "1"
// "Kye2" , value type -> GetType -> Dictionary -> DFS-> recursive -> pass extra argument
// Key2 -> prefix 
// prefix is empty? key : (key is empty)? prefix : (prefix +"." + key) 
//