In-place reverse a list in Kotlin

This article explores different ways to in-place reverse a list in Kotlin.

The conversion should be done in-place, i.e., without using any auxiliary space by overwriting the existing elements of the specified list.

1. Using `Collections.reverse()` function

To in-place reverse the order of the elements in a list, you can use the Collections.reverse() function. It takes the list whose elements are to be reversed.

import java.util.Collections

fun main() {

val days: List<String> = listOf("Sunday", "Monday", "Friday")

Collections.reverse(days)

println(days) // [Friday, Monday, Sunday]

}

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2. Using `reversed()` function

You can also use the native reversed() function to reverse a list, but the conversion won’t be in-place, and it creates a new list.

fun main() {

val days: List<String> = listOf("Sunday", "Monday", "Friday")

val reversedList = days.reversed();

println(reversedList) // [Friday, Monday, Sunday]

}

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If you just need the view of the list in reverse order, you can get one using the asReversed() function.

fun main() {

val days: List<String> = listOf("Sunday", "Monday", "Friday")

val reversedView = days.asReversed();

println(reversedView) // [Friday, Monday, Sunday]

}

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2. Using `add()` with `removeLast()` function

Another plausible way to in-place reverse a list is to reorder the elements within the list. We can do this by removing an element from the end of the list and insert it into the very beginning, one at a time, using a for-loop.

fun main() {

val days: MutableList<String> = mutableListOf("Sunday", "Monday", "Friday")

for (i in days.indices) {

days.add(i, days.removeLast())

}

println(days) // [Friday, Monday, Sunday]

}

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3. Using Recursion

Finally, you can use recursion to in-place reverse a list. Here’s a recursive solution:

fun <T> reverseList(list: MutableList<T>?) {

// base case: the list is empty, or only one element is left

if (list == null || list.size <= 1) {

return

}

// remove the first element

val value = list.removeFirst()

// recur for remaining items

reverseList(list)

// insert the top element back after recur for remaining items

list.add(value)

}

fun main() {

val days: MutableList<String> = mutableListOf("Sunday", "Monday", "Friday")

reverseList(days)

println(days) // [Friday, Monday, Sunday]

}

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That’s all about in-place reversing a list in Kotlin.

In-place reverse a list in Kotlin

1. Using Collections.reverse() function

2. Using reversed() function

2. Using add() with removeLast() function

3. Using Recursion

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1. Using `Collections.reverse()` function

2. Using `reversed()` function

2. Using `add()` with `removeLast()` function