代码优化：bottom 函数

优化以下代码：
function bottom(layout: Layout): number {
  let max = 0;
  let bottomY;
  for (let i = 0, len = layout.length; i < len; i += 1) {
    bottomY = layout[i].y + layout[i].h;
    if (bottomY > max) {
      max = bottomY;
    }
  }
  return max;
}