How to use R to split a string so that the following desired result can be achieved?

```
"A++" => "A" "" ""
"A+B+" => "A" "B" ""
"A+B+C" => "A" "B" "C"
"A++C" => "A" "" "C"
"++C" => "" "" "C"

```

I tried the `strsplit()`, the result of `strsplit("A++","\\+")[[1]]` is `"A" ""`, missing one `""`.