_findNode(root, width, height) {
    if (root.used) {
      return this._findNode(root.right, width, height) || this._findNode(root.down, width, height);
    }

    if (width <= root.width && height <= root.height) {
      return root;
    }

    return null;
  }
Find a node

@param {object} root         Root
@param {number} width        Width
@param {number} height       Height
@returns {object | null}     Node