code: function height(node) {
    return node ? (1 + Math.max(height(node.left), height(node.right))) : 0;
  }
docs: The function Compute the 'height' of a tree.
  Height is the number of nodes along the longest path
  from the root node down to the farthest leaf node.
  
  @param  {Node} node
  @return {Number}