BFS traversal

time complexity O(n), since there are n nodes.

space complexity O(n), since the last layer has n/2 node in a full binary tree.

```
public class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> result = new ArrayList<>();
if (root == null) {
return result;
}
Deque<TreeNode> queue = new LinkedList<>();
queue.addLast(root);
while (!queue.isEmpty()) {
int size = queue.size();
List<Integer> layer = new ArrayList<>();
while (size > 0) {
TreeNode cur = queue.pollFirst();
layer.add(cur.val);
if (cur.left != null) {
queue.addLast(cur.left);
}
if (cur.right != null) {
queue.addLast(cur.right);
}
size--;
}
result.add(layer);
}
return result;
}
}
```