https://leetcode.com/problems/binary-tree-right-side-view/
Given the root of a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.
Input: root = [1,2,3,null,5,null,4]
Output: [1,3,4]
Input: root = [1,null,3]
Output: [1,3]
Input: root = [] Output: []