解题思路

class Solution {
    private LinkedList<List<Integer>> ans;
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        ans = new LinkedList<>();
        getOrder(root, 1);
        return ans;
    }
    private void getOrder(TreeNode root, int level) {
        if(root == null)    return;
        if(ans.size() < level)
            ans.addFirst(new LinkedList<Integer>());
        List<Integer> subList = ans.get(ans.size() - level);
        subList.add(root.val);
        getOrder(root.left, level + 1);
        getOrder(root.right, level + 1);
    }
}