rightSideView = function(root, level = 0, rightSide = []) {
    const isBaseCase = root === null;
    if (isBaseCase) return rightSide;

    const isLastNode = level === rightSide.length
    if (isLastNode) rightSide.push(root.val);

    return dfs(root, level, rightSide)
}
https://leetcode.com/problems/binary-tree-right-side-view/
Time O(N) | Space O(H)
@param {TreeNode} root
@return {number[]}