Flatten dictionary - recursive - my practice on March 8, 2018 8:00 PM mock interview; it took me less than 20 minutes to work on.

FlattenDictionary_recursive_MyPractice.cs

using System;
using System.Collections.Generic;

class Solution
{
  public static Dictionary<string, string> FlattenDictionary(Dictionary<string, object> dict)
  {
    if(dict == null)
      return new Dictionary<string, string>(); 
    
    var flatten = new Dictionary<string, string>(); 
    
    foreach(var pair in dict)
    {
      var key   = pair.Key; 
      var value = pair.Value; 
      
      if(value.GetType() == typeof(Dictionary<string, object>))
      {
        var childDict = FlattenDictionary((Dictionary<string, object>)value);
        
        foreach(var item in childDict)
        {
          var childKey   = item.Key; 
          var childValue = item.Value; 
          
          var newKey = key + "." + childKey; 
          if(string.IsNullOrEmpty(key))
            newKey = childKey; 
          
          if(string.IsNullOrEmpty(childKey))
            newKey = key; 
          
          flatten.Add(newKey, childValue); 
        }        
      }
      else 
      {
        flatten.Add(key, (string)value); 
      }
    }
    
    return flatten; 
  }

  static void Main(string[] args)
  {

  }
}

/*
value is not dictionary -> key, value to flatten dictionary
value is dictionary -> recursive, 
  value -> flattenDictChild(string, string)
    
key -> prefix 
go over each entry in flattenDictChild, 
  foreach pair in dict
  
    put key = prefix + "." + pair.key 
        value = pair.value
    
return flatten dictionary 

DFS/ recursive -> 
*/