[LeetCode] 102. 二叉树的层序遍历

public List<List<Integer>> levelOrder(TreeNode root) {
    List<List<Integer>> res = new ArrayList<>();

    Queue<TreeNode> queue = new ArrayDeque<>();
    if (root != null) {
        queue.add(root);
    }
    while (!queue.isEmpty()) {
        int n = queue.size();
        List<Integer> level = new ArrayList<>();
        for (int i = 0; i < n; i++) { 
            TreeNode node = queue.poll();
            level.add(node.val);
            if (node.left != null) {
                queue.add(node.left);
            }
            if (node.right != null) {
                queue.add(node.right);
            }
        }
        res.add(level);
    }

    return res;
}