我对R相当陌生，看起来这应该很简单，但我的搜索一无所获。
我有一些数据在Excel中格式化为一系列列表-即：第1行包含类别，项目如下所示：

Fruit Vegetable  Grain    Meat
1  Apple    Carrot  Wheat Chicken
2  Grape    Potato   Rice    Beef
3 Banana           Barley    Pork
4 Orange                         
5   Kiwi

字符串
我想把它变成一个长的 Dataframe ，列名变成这样的类别：

Category    Food
1      Fruit   Apple
2      Fruit   Grape
3      Fruit  Banana
4      Fruit  Orange
5      Fruit    Kiwi
6  Vegetable  Carrot
7  Vegetable  Potato
8      Grain   Wheat
9      Grain    Rice
10     Grain  Barley
11      Meat Chicken
12      Meat    Beef
13      Meat    Pork

型
下面是输入数据：

df <- structure(list(Fruit = c("Apple", "Grape", "Banana", "Orange", 
"Kiwi"), Vegetable = c("Carrot", "Potato", NA, NA, NA), Grain = c("Wheat", 
"Rice", "Barley", NA, NA), Meat = c("Chicken", "Beef", "Pork", 
NA, NA)), row.names = c(NA, -5L), class = c("tbl_df", "tbl", 
"data.frame"))

型
我觉得我应该使用tidyr：：pivot_longer（），但不太清楚如何使用。
任何建议赞赏！