code: function rotateLeft (node) {
    var rightNode = node.right;
    node.right    = rightNode.left;
  
    if (rightNode.left) rightNode.left.parent = node;
  
    rightNode.parent = node.parent;
    if (rightNode.parent) {
      if (rightNode.parent.left === node) {
        rightNode.parent.left = rightNode;
      } else {
        rightNode.parent.right = rightNode;
      }
    }
  
    node.parent    = rightNode;
    rightNode.left = node;
  
    node.balanceFactor += 1;
    if (rightNode.balanceFactor < 0) {
      node.balanceFactor -= rightNode.balanceFactor;
    }
  
    rightNode.balanceFactor += 1;
    if (node.balanceFactor > 0) {
      rightNode.balanceFactor += node.balanceFactor;
    }
    return rightNode;
  }
docs: Single left rotation
  @param  {Node} node
  @return {Node}