_find(value, current) {
    if (!current) {
      return null;
    }

    let node;
    if (current.value === value) {
      node = current;
    } else if (current.value > value) {
      node = this._find(value, current.left);
    } else if (current.value < value) {
      node = this._find(value, current.right);
    }

    return node;
  }
Finds a node by its value in the given sub-tree.