How to reverse a string using recursion in Java

Recursion in Java is a process where a function/method calls itself consistently. In the programming language, if a program permits us to call a method inside a similar method name, it is known as a recursive call. It makes the code minimal, yet it is challenging to comprehend. It is utilized to tackle a complex problem that can be broken into more modest monotonous issues. Utilizing recursion, we can take care of numerous mind-boggling issues like the factorial program, Fibonacci series, Towers of Hanoi etc.

Methodology:

To start with, eliminate the first element/ character from the string and append that element/ character toward the finish of the string.
Repeat the above step until the info string becomes null.

StringReverse.java

// Java Program to Reverse a String using recursion
// importing the required packages
import java.io.*;
import java.util.*;
class StringReverse {
    // This method is used to reverse a string using stack
	public static String reverseString(String str)
	{
        // Recursive method for revarsinga a string
        if(str.isEmpty())  
            {  
                // If the string satisfy the above condition we simply return the same string
                return str;  
            }   
            else   
            {  
                return reverseString(str.substring(1))+str.charAt(0);  
            }  
	}
	// Driver code
	public static void main(String[] args)
	{
	    System.out.println(" Hello, World! ");
	    // Creating an object to the Scanner class such that we can take the inputs from the user
	    Scanner sc = new Scanner(System.in);
	    System.out.println(" Enter a String: ");
	    // reading the String
	    String str = sc.next();
	    // printing the string
	    System.out.println(" The entered String is: " + str);
	    // printing the reverse String
	    System.out.println(" The String in reverse order is: " + reverseString(str));
	}
}

Output:

How to reverse a string using recursion in Java

Explanation:

In the above program, we have made a method named reverseString(). It parses our null or not. Assuming that the string is vacant, it returns the same string. We have utilized the accompanying two methods from the String class:

Time complexity: O(n^2)

O(n^2) because substr() has the time complexity of O(k) where k is the size of the brought string back. So for each recursive call, we are decreasing the size of the string by one, which prompts a series like (k-1)+(k-2)+… +1 = k*(k-1)/2 = O(k^2) = O(n^2)

Space complexity: O(n)

Where n is the size of the string.

Efficient Approach:

We can store each person in the recursive stack and afterwards can print while returning as displayed in the underneath code:

// Java Program to Reverse a String using recursion
// importing the required packages
import java.io.*;
import java.util.*;
class ReverseString {
    static void stringReverse(String str, int index, int n)
    {
        // returns to the method if the index has reached at the end of the string
        if (index == n) 
        {
            return;
        }
        char temp = str.charAt(index); // storing away each character beginning from index 0 in method call stack;
        stringReverse(str, index + 1, n); //raising index when every time when running a recursive function
        System.out.print(temp); // printing each stored character while recurring back
  }
	// Driver code
	public static void main(String[] args)
	{
	    System.out.println(" Hello, World! ");
	    // Creating an object to the Scanner class such that we can take the inputs from the user
	    Scanner sc = new Scanner(System.in);
	    System.out.println(" Enter a String: ");
	    // reading the String
	    String str = sc.next();
	    // printing the string
	    System.out.println(" The entered String is: " + str);
	    int n = str.length();
	    // printing the reverse String
	    System.out.println(" The String in reverse order is: ");
	    stringReverse(str, 0, n);
	}
}

Output: