思路

层次优先遍历

code

 public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new LinkedList<>();
        levelMaker(res, root, 0);
        return res;
    }
    public void levelMaker(List<List<Integer>> list,TreeNode root,int level){
        if(root==null)
            return;
        if(level>=list.size())
            list.add(0,new LinkedList<Integer>());
            levelMaker(list,root.left,level+1);
            levelMaker(list,root.right,level+1);
            list.get(list.size()-level-1).add(root.val);
    }