Use the below simple code. 
 public void CreateFolder(string url, string foldername,string documentlibname)
 {
 try
 {
 ClientContext clientContext = new ClientContext(url);
 Web web = clientContext.Web;
 var query = clientContext.LoadQuery(web.Lists.Where(p => p.Title == documentlibname));## Heading ##
 clientContext.ExecuteQuery();
 List list = query.FirstOrDefault();
 var folder = list.RootFolder;
 clientContext.Load(folder);
 clientContext.ExecuteQuery();
 string[] PathElements = foldername.Split(new char[] { '/' }, StringSplitOptions.RemoveEmptyEntries);
 for (int i = 0; i < PathElements.Length; i++)
 {
 folder = folder.Folders.Add(PathElements[i].ToString());
 clientContext.ExecuteQuery();
 }
 

 
 //folder = folder.Folders.Add(foldername);
 //clientContext.ExecuteQuery();
 }
 catch (System.Exception ex)
 {
 
 throw;
 }
 }