BFS traverses entire tree.

Each loop only add the first element of queue, and push right node at first.

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