class Solution {
    public List<List<Integer>> resList = new ArrayList<List<Integer>>();
    
    public List<List<Integer>> levelOrder(TreeNode root) {
        checkFun(root);
        return resList;

    }

    public void checkFun(TreeNode node) {
        if (node == null) {
            return;
        }
        Queue<TreeNode> que = new LinkedList<TreeNode>();
        que.offer(node);

        while(!que.isEmpty()) {
            List<Integer> itemList = new ArrayList<>();
            int len = que.size();

            while (len>0) {
                TreeNode tmpNode = que.poll();
                itemList.add(tmpNode.val);

                if (tmpNode.left != null) {
                    que.offer(tmpNode.left);
                }
                if (tmpNode.right != null) {
                    que.offer(tmpNode.right);
                }
                len--;
            }
            resList.add(itemList);
        }
    }
}

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        invertTree(root.left);
        invertTree(root.right);
        swapChildren(root);
        return root;
    }

    private void swapChildren(TreeNode root) {
        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;
    }
}

class Solution {
    private boolean isEquivalent(TreeNode left, TreeNode right) {
        if (left==null || right==null) return left == right;
        else return left.val == right.val && isEquivalent(left.left, right.right) && isEquivalent(left.right, right.left);
    }

    public boolean isSymmetric(TreeNode root) {
        return isEquivalent(root.left, root.right);
    }
}