code: function lookup(tree, key) {
    if (!tree) {
      return null
    }
  
    switch (compare(tree.key, key)) {
      case 0:
        return tree
      case 1:
        return lookup(tree.right, key)
      case -1:
        return lookup(tree.left, key)
      default:
        break
    }
  }
docs: search `key` in AVL tree
  @param tree {Object}
  @param key {Buffer|String}
  @returns {Buffer|Object|null}