二叉树层序遍历Java版

public List<List<Integer>> levelOrder(TreeNode root) { 
   
    List<List<Integer>> result = new ArrayList<>();
    if(root == null) return result;
    List<TreeNode> queue = new ArrayList<>();
    queue.add(root);
    while(queue.size() != 0){ 
   
        int n = queue.size();
        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < n; i++) { 
   
            TreeNode node = queue.remove(0);
            list.add(node.val);
            if(node.left != null)
                queue.add(node.left);
            if(node.right != null)
                queue.add(node.right);
        }
        result.add(list);
    }
    return result;
}