const isBalanced = function(root) {
    if (!root) return true

    if (Math.abs(getHeight(root.left) - getHeight(root.right)) > 1) {
        return false
    }

    return isBalanced(root.left) && isBalanced(root.right)
    
    function getHeight (root) {
        if (!root) return 0

        return Math.max(getHeight(root.left), getHeight(root.right)) + 1
    }
}