The idea is simple: use BFS to visit tree layer by layer, just like a normal BFS. Use queue size to know if the current TreeNode is the right most TreeNode in current layer. If it is, add it to the result list.

```
if (root == null) {
return new ArrayList<Integer>();
}
List<Integer> res = new ArrayList<>();
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()) {
int size = queue.size();
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
if (i == size - 1) {
res.add(node.val);
}
if (node.left != null) {
queue.offer(node.left);
}
if (node.right != null) {
queue.offer(node.right);
}
}
}
return res;
```