code: private Node rotateLeft(Node node) {
          Node leftChild = node.left;
          node.left = leftChild.right;
          leftChild.right = node;
          node.height = calculateHeight(getHeight(node.left), getHeight(node.right));
          leftChild.height = calculateHeight(getHeight(leftChild.left), node.height);
          return leftChild;
      }
docs: Rotates the given binary tree node with the left child. At the end of this operation:
  * The Left Child's right node is the Given Node
  * The Given Node's Left Child is the Left Child's original Right Child.