Picture a binary tree drawn top-down and look at it from the right-hand side. On each level you see only the node furthest to the right; every other node on that level is hidden behind it.
Given the root of a binary tree, return the values you can see, one per level, from the top level down. An empty tree shows nothing, so its answer is an empty list.
9 hides 3 on the second level. The third level has no right-hand branch at all, so the rightmost node there is 5, inside the left subtree, and 4 below it is visible too.
3 is a left child, but it is alone on its level, so nothing hides it.
Constraints
0 and 100 nodes.-100 <= Node.val <= 100The answer holds one value per level: the last node on that level when you read it left to right.