private fun remove(node: TreeNode<K, V>?, key: K?): TreeNode<K, V>? {
        val keyHashCode = key.hashCode()
        val nodeHasCode = node?.key?.hashCode() ?: 0

        if (keyHashCode < nodeHasCode) {
            node?.left = remove(node?.left, key)
        } else if (keyHashCode > nodeHasCode) {
            node?.right = remove(node?.right, key)
        } else {
            if (node?.left == null && node?.right == null) {
                return null
            } else if (node.left == null) {
                return node.right
            } else if (node.right == null) {
                return node.left
            } else {
                val minNode = minValue(node.right)
                node.key = minNode?.key
                node.value = minNode?.value
                node.right = remove(node.right, minNode?.key)
            }
        }

        return node
    }

    private fun minValue(node: TreeNode<K, V>?): TreeNode<K, V>? {
        if (node?.left != null) {
            return minValue(node.left)
        }

        return node
    }