102. 二叉树的层序遍历

class Solution {
    List<List<Integer>> res = new ArrayList<>();
    public List<List<Integer>> levelOrder(TreeNode root) {  
        dfs(root,1);
        return res;
    }
    public void dfs(TreeNode root, int level) {
        if(root == null) return;
        if(res.size() < level) res.add(new ArrayList<>());
        res.get(level-1).add(root.val);
        dfs(root.left,level+1);
        dfs(root.right,level+1);

    }
}