class  {
    public boolean isBalanced(TreeNode root) {
        if (root == null)
            return true;
        int left = height(root.left);
        int right = height(root.right);
        if (left == -1 || right == -1)
            return false;
        if (Math.abs(left - right) <= 1) {
            return true;
        }
        return false;
    }

    Integer height(TreeNode root) {
        if (root == null)
            return 0;
        int left = height(root.left);
        int right = height(root.right);
        if (left == -1 || right == -1)
            return -1;
        if (Math.abs(left - right) <= 1)
            return Math.max(left, right) + 1;
        else
            return -1;
    }
}