Kotlin Program to Reverse a String with Example

You can reverse a string in Kotlin using val reversed = original.reversed() which returns the reversed string.

Examples

Inputhello

Outputolleh

InputKotlin

OutputniltoK

Input

Output

How to Think About It

To reverse a string, think of reading the characters from the end to the start and putting them in a new string. Kotlin provides a simple function reversed() that does this for you, so you just call it on the original string.

Algorithm

1

Get the input string from the user or predefined variable.

2

Use the built-in <code>reversed()</code> function to reverse the string.

3

Store the reversed string in a new variable.

4

Print or return the reversed string.

Code

kotlin

fun main() {
    val original = "hello"
    val reversed = original.reversed()
    println(reversed)
}

Output

olleh

Dry Run

Let's trace the string "hello" through the code

1

Original string

original = "hello"

2

Reverse string using reversed()

reversed = "hello".reversed() -> "olleh"

3

Print reversed string

Output: olleh

Index	Character	Reversed Position
0	h	4
1	e	3
2	l	2
3	l	1
4	o	0

Why This Works

Step 1: Using reversed() function

The reversed() function creates a new string by reading the original string from the last character to the first.

Step 2: Storing the result

We store the reversed string in a new variable so we can use or print it later.

Step 3: Printing the reversed string

Finally, we print the reversed string to show the output.

Alternative Approaches

Manual loop reversal

kotlin

fun main() {
    val original = "hello"
    var reversed = ""
    for (i in original.length - 1 downTo 0) {
        reversed += original[i]
    }
    println(reversed)
}

This method uses a loop to build the reversed string manually but is less efficient due to string concatenation.

Using StringBuilder reverse()

kotlin

fun main() {
    val original = "hello"
    val reversed = StringBuilder(original).reverse().toString()
    println(reversed)
}

This uses StringBuilder's reverse method which is efficient and mutable, good for longer strings.

Complexity: O(n) time, O(n) space

Time Complexity

The reversal reads each character once, so it takes linear time proportional to the string length.

Space Complexity

A new string is created to hold the reversed characters, so space is also linear in the string length.

Which Approach is Fastest?

Using reversed() or StringBuilder.reverse() is faster and more efficient than manual concatenation loops.

Approach	Time	Space	Best For
reversed()	O(n)	O(n)	Simple and clean code
StringBuilder.reverse()	O(n)	O(n)	Efficient for large strings
Manual loop with concatenation	O(n²)	O(n)	Educational but inefficient