Binary Tree Right Side View

Imagine standing on the right side of the tree. Print the values you can see, from top to bottom — that is, the last node of every level.

Tree format. A binary tree is given on one line in level order (top to bottom, left to right), with null for a missing child — the same way most interview platforms show trees. Trailing nulls are left out, and an empty tree is the single word null. Example: 3 9 20 null null 15 7 is

    3
   / \
  9   20
     /  \
    15   7

Input: one tree.

Output: the visible values on one line, or empty for an empty tree.

Constraints: 0 ≤ nodes ≤ 10⁴. The tree has at most 10⁴ nodes and its height is at most 500, so recursion is safe in every language.

Watch out: the rightmost node of a level is not always a right child — 1 2 3 4 shows 4, which hangs on the left.