The function in Listing 20 returns a vector of vectors. This is because each document could generate zero to many map entries, and each map entry is represented as a vector.
清单20中的函数返回矢量的矢量，这是因为每个文档都可能生成零到多个映射条目，而每个映射条目都被表示为一个矢量。
youdao
Many Map Zoom levels.
许多地图缩放等级。
应用推荐
模块上移
模块下移
不移动