Realize first-in first-out with bfs thought queue
public int[] levelOrder(TreeNode root) {
if (root == null) return new int[0];
ArrayList<Integer> list = new ArrayList<>();
Deque<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()){
TreeNode node = queue.pop();
list.add(node.val);
if (node.left != null) queue.offer(node.left);
if (node.right != null) queue.offer(node.right);
}
int[] res = new int[list.size()];
for (int i = 0; i < list.size(); i++)
res[i] = list.get(i);
return res;
}