Swift Program to Reverse String using Recursion

Swift – Reverse String using Recursion

In this tutorial, we will learn how to reverse a string using recursion in Swift. We will understand the concept of recursion, write an algorithm, and implement it step by step with a complete Swift program and output.

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What is Recursion?

Recursion is a technique in which a function calls itself to solve smaller subproblems of a larger problem. It is widely used in algorithms where problems can be divided into similar subproblems. A recursive function must have:

• A base case that stops the recursion.
• A recursive case that reduces the problem size and calls itself.

To reverse a string recursively, we can divide the problem into smaller parts and work on the first character and the rest of the string recursively.

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Algorithm to Reverse a String Using Recursion

1. Take the string as input.
2. Base Case: If the string is empty or has one character, return the string itself.
3. Recursive Case:
   1. Take the first character of the string.
   2. Reverse the rest of the string by recursively calling the function.
   3. Append the first character to the reversed substring.

This algorithm ensures that the string is broken down into smaller parts until the base case is reached.

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Step-by-Step Implementation in Swift

1. Define the Base Case

Create a function that returns the string itself when its length is 0 or 1:

func reverseString(_ str: String) -> String {
    if str.count <= 1 {
        return str // Base case: return the string if it's empty or has one character
    }
    return "" // Placeholder for the recursive case
}

2. Add the Recursive Case

Implement the logic to reverse the string by appending the first character to the reversed substring:

func reverseString(_ str: String) -> String {
    if str.count <= 1 {
        return str // Base case
    }
    let firstChar = String(str.first!) // Get the first character
    let restOfString = String(str.dropFirst()) // Get the rest of the string
    return reverseString(restOfString) + firstChar // Recursive call
}

Explanation:

if str.count <= 1: Stops the recursion when the string is empty or has one character.

String(str.first!): Extracts the first character of the string.

String(str.dropFirst()): Removes the first character from the string.

reverseString(restOfString) + firstChar: Recursively reverses the rest of the string and appends the first character at the end.

3. Test the Function

Let’s test the function with some example inputs:

// Test cases
print("Reversed 'hello': \(reverseString("hello"))")  // Output: "olleh"
print("Reversed 'Swift': \(reverseString("Swift"))")  // Output: "tfiwS"
print("Reversed 'A': \(reverseString("A"))")          // Output: "A"
print("Reversed '': \(reverseString(""))")            // Output: ""

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Complete Swift Program

Here’s the complete Swift program:

import Foundation

// Recursive function to reverse a string
func reverseString(_ str: String) -> String {
    if str.count <= 1 {
        return str // Base case
    }
    let firstChar = String(str.first!) // Get the first character
    let restOfString = String(str.dropFirst()) // Get the rest of the string
    return reverseString(restOfString) + firstChar // Recursive call
}

// Test cases
print("Reversed 'hello': \(reverseString("hello"))")  // Output: "olleh"
print("Reversed 'Swift': \(reverseString("Swift"))")  // Output: "tfiwS"
print("Reversed 'A': \(reverseString("A"))")          // Output: "A"
print("Reversed '': \(reverseString(""))")            // Output: ""

Output:

Reversed 'hello': olleh
Reversed 'Swift': tfiwS
Reversed 'A': A
Reversed '': 
Program ended with exit code: 0

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