Binary Tree Level Order Traversal

1public List<List<Integer>> levelOrder(TreeNode root) {
2    List<List<Integer>> result = new ArrayList<>();
3
4    if (root == null) {
5        return result;
6    }
7
8    Queue<TreeNode> queue = new LinkedList<>();
9    queue.offer(root);
10
11    while (!queue.isEmpty()) {
12        int levelSize = queue.size();
13        List<Integer> currentLevel = new ArrayList<>();
14
15        for (int i = 0; i < levelSize; i++) {
16            TreeNode node = queue.poll();
17            currentLevel.add(node.val);
18
19            if (node.left != null) {
20                queue.offer(node.left);
21            }
22            if (node.right != null) {
23                queue.offer(node.right);
24            }
25        }
26
27        result.add(currentLevel);
28    }
29
30    return result;
31}