size: retorna la cantidad total de nodos del árbol

function size(tree) {
  if (tree.left && tree.right) {
    return 1 + size(tree.left) + size(tree.right)
  } else if (tree.left) {
    return 1 + size(tree.left)
  } else if (tree.right) {
    return 1 + size(tree.right)
  } else {
    return 1
  }
}