Problem

Given the root of a Binary Search Tree (BST), return it's content serialized as a string.

Example 1

serializeBST([2,1,3,null,null,null,4])
// return [2,1,3,null,null,null,4]

Example 2

serializeBST([3,1,null,null,2])
// return [3,1,null,null,2]