题目

代码

if(root == null){
return new ArrayList<>();
}

    Queue<TreeNode> queue = new LinkedList<>();
    List<List<Integer>> res = new ArrayList<>();

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

    return res;