public boolean isBalanced(TreeNode root) {
    // base case
    if (root == null) {
        return true;
    }
    int leftHeight = GetHeight(root.left);
    int rightHeight = GetHeight(root.right);
    if (Math.abs(leftHeight - rightHeight) > 1) {
        return false;
    }
    return isBalance(root.left) && isBalance(root.right);
}