Kotlin. ArrayList, how to move element to first position - algorithm

Question

I have a list of Lessons. Here is my Lessons class:
data class Lessons(
val id: Long,
val name: String,
val time: Long,
val key: String
)
I need to move the element to the beginning of the list, whose key field has a value "priority".
Here is my code:
val priorityLesson = lessons.find { it.key == "priority" }
if (priorityLesson != null) {
lessons.remove(priorityLesson)
lessons.add(0, priorityLesson)
}
Everything is working but I do not like this solution, perhaps there is a more efficient way to perform this algorithm. In addition, it comes to me to convert the list to mutable, and I would like to leave it immutable.
Please help me.

Answer 1

One way is to call partition() to split the list into a list of priority lesson(s), and a list of non-priority lessons; you can then rejoin them:
val sorted = lessons.partition{ it.key == "priority" }
.let{ it.first + it.second }
As well as handling the case of exactly one priority lesson, that will cope if there are none or several. And it preserves the order of priority lessons, and the order of non-priority lessons.
(That will take a little more memory than modifying the list in-place; but it scales the same — both are 𝒪(n). It's also easier to understand and harder to get wrong!)

Answer 2

First, I would call your class Lesson rather than Lessons as it represents a single lesson. Your choice of the variable name lessons is good for your list of lessons.
You can use a mutable list and move the item to the top:
val priorityLessonIndex = lessons.indexOf { it.key == "priority" }
if (priorityLessonIndex != -1)
lessons[0] = lessons[priorityLessonIndex]
.also { lessons[priorityLessonIndex] = lessons[0] }
Or you can use an immutable list:
val priorityLesson = lessons.firstOrNull { it.key == "priority" }
val newList =
if (priorityLesson != null)
listOf(priorityLesson) + (lessons - priorityLesson)
else
lessons
A possibly more efficient way, which avoids creation of intermediate lists:
val newList = buildList(lessons.size) {
lessons.filterTo(this) { it.key == "priority" }
lessons.filterTo(this) { it.key != "priority" }
}