/**
Given the root of a binary tree, flatten the tree into a "linked list": 
 
 The "linked list" should use the same T
                            
                        
                        
                            
                                2022-08-23
                            
                            
                        
                     
                            
                             
                            
                             
                            
                             
                            
                             
                            
                             
                            
                             
                            
                             
                            
                             
                            
                            