102. 二叉树的层序遍历

public List<List<Integer>> levelOrder(TreeNode root) {
    if (root == null) {
        return new ArrayList<>();
    }

    List<List<Integer>> result = new ArrayList<>();

    Queue<TreeNode> queue = new ArrayDeque<>();
    queue.add(root);
    while (!queue.isEmpty()) {
        List<Integer> level = new ArrayList<>();
        int queueSize = queue.size();
        for (int i = 0; i < queueSize; i++) {
            TreeNode temp = queue.poll();
            level.add(temp.val);
            //左子树不为空，队列中添加左
            if (temp.left != null) {
                queue.add(temp.left);
            }
            //右子树不为空，队列中添加右
            if (temp.right != null) {
                queue.add(temp.right);
            }
        }
        result.add(level);
    }

    return result;
}