Program to reverse a string (Iterative and Recursive)

Given a string, write a recursive program to reverse it.

Method 1 (Using Stack)

C++

// C++ program to reverse a string using stack
#include <bits/stdc++.h>
using namespace std;

void recursiveReverse(string &str)
{
   stack<char> st;
   for (int i=0; i<str.length(); i++)
       st.push(str[i]);

   for (int i=0; i<str.length(); i++) {
       str[i] = st.top();
       st.pop();
   }
}

// Driver program
int main()
{
    string str = "geeksforgeeks";
    recursiveReverse(str);
    cout << str;
    return 0;
}

Java

// Java program to reverse a string using stack
import java.util.*;
class GFG
{
  public static String recursiveReverse(char []str)
 {
   Stack<Character> st = new Stack<>();
   for(int i=0; i<str.length; i++)
        st.push(str[i]);

   for (int i=0; i<str.length; i++) {
    str[i] = st.peek();
    st.pop();
   }
   return String.valueOf(str);// converting character array to string
 }

// Driver program
   public static void main(String []args)
   {
      String str = "geeksforgeeks";
      str = recursiveReverse(str.toCharArray());// passing character array as parameter
      System.out.println(str);
   }
}
// This code is contributed by Adarsh_Verma

Python3

# Python program to reverse a string using stack

def recursiveReverse(str):

    # using as stack
    stack = []

    for i in range(len(str)):
        stack.append(str[i])

    for i in range(len(str)):
        str[i] = stack.pop()

if __name__ == "__main__":
    str = "geeksforgeeks"

    # converting string to list
    # because strings do not support
    # item assignment
    str = list(str)
    recursiveReverse(str)

    # converting list to string
    str = ''.join(str)
    print(str)

# This code is contributed by
# sanjeev2552

C#

// C# program to reverse a string using stack
using System;
using System.Collections.Generic;

class GFG
{
    public static String recursiveReverse(char []str)
    {
        Stack<char> st = new Stack<char>();
        for(int i = 0; i < str.Length; i++)
            st.Push(str[i]);

        for (int i = 0; i < str.Length; i++)
        {
            str[i] = st.Peek();
            st.Pop();
        }

        // converting character array to string
        return String.Join("",str);
    }

    // Driver program
    public static void Main()
    {
        String str = "geeksforgeeks";

        // passing character array as parameter
        str = recursiveReverse(str.ToCharArray());
        Console.WriteLine(str);
    }
}

// This code is contributed by Rajput-Ji

Javascript

<script>
      // JavaScript program to reverse a string

      function recursiveReverse(str)
      {
        var revString = "";

        for (var i = str.length - 1; i >= 0; i--)
        {
          revString += str[i];
        }

        return revString;
      }

      // Driver program
      var str = "geeksforgeeks";
      document.write(recursiveReverse(str));

      // This code is contributed by rdtank.
    </script>

Output:

skeegrofskeeg

Time complexity : O(n) 
Auxiliary Space : O(n)

Method 2 (Iterative)

C++

// A Simple Iterative C++ program to reverse
// a string
#include <bits/stdc++.h>
using namespace std;

// Function to reverse a string
void reverseStr(string& str)
{
    int n = str.length();

    // Swap character starting from two
    // corners
    for (int i = 0; i < n / 2; i++)
        swap(str[i], str[n - i - 1]);
}

// Driver program
int main()
{
    string str = "geeksforgeeks";
    reverseStr(str);
    cout << str;
    return 0;
}

Java

// A Simple Java program
// to reverse a string
import java.util.Scanner;

public class reverseStr
{
// Function to reverse
// a string
void stringReverse()
{
    String str = "geeksforgeeks";
    int length = str.length();
    StringBuffer revString = new StringBuffer();
    for (int i = length - 1;
             i >= 0; i--)
    {
        revString.append(str.charAt(i));
    }
    System.out.println(revString);
}

// Driver Code
public static void main(String []args)
{
    reverseStr s= new reverseStr();
    s.stringReverse();
}
}

// This code is contributed
// by prabhat kumar singh

Python

# A Simple python program
# to reverse a string

# Function to
# reverse a string
def reverseStr(str):
    n = len(str)

    # initialising a empty
    # string 'str1'
    str1 = ''
    i = n - 1
    while i >= 0:

        # copy str
        # to str1
        str1 += str[i]
        i -= 1
    print(str1)

# Driver Code
def main():
    str = "geeksforgeeks";
    reverseStr(str);

if __name__=="__main__":
    main()

# This code is contributed
# by prabhat kumar singh

C#

// A Simple Iterative C# program to reverse
// a string
using System;

class GFG
{

// Function to reverse a string
static String reverseStr(String str)
{
    int n = str.Length;

    // Swap character starting from two
    // corners
    for (int i = 0; i < n / 2; i++)
        str = swap(str,i,n - i - 1);
    return str;
}
static String swap(String str, int i, int j)
{
    char []ch = str.ToCharArray();
    char temp = ch[i];
    ch[i] = ch[j];
    ch[j] = temp;
    return String.Join("",ch);
}

// Driver code
public static void Main(String[] args)
{
    string str = "geeksforgeeks";
    str= reverseStr(str);
    Console.WriteLine(str);
}
}

// This code is contributed by Princi Singh

PHP

<?php
// A Simple Iterative PHP
// program to reverse
// a string

// Function to reverse a string
function reverseStr(&$str)
{
    $n = strlen($str);

    // Swap character starting
    // from two corners
    for ($i = 0; $i < $n / 2; $i++)
        //swap the string
        list($str[$i],
             $str[$n - $i - 1]) = array($str[$n - $i - 1],
                                        $str[$i]);
}

// Driver Code
$str = "geeksforgeeks";

reverseStr($str);
echo $str;

// This code is contributed by ajit
?>

Javascript

<script>
    // A Simple Iterative Javascript program to reverse a string

    // Function to reverse a string
    function reverseStr(str)
    {
        let n = str.length;

        // Swap character starting from two
        // corners
        for (let i = 0; i < parseInt(n / 2, 10); i++)
            str = swap(str,i,n - i - 1);
        return str;
    }

    function swap(str, i, j)
    {
        let ch = str.split('');
        let temp = ch[i];
        ch[i] = ch[j];
        ch[j] = temp;
        return ch.join("");
    }

    let str = "geeksforgeeks";
    str= reverseStr(str);
    document.write(str);

</script>

Output:

skeegrofskeeg

Time complexity : O(n) 
Auxiliary Space : O(1)

Method 3 (Iterative using two pointers)

C++

// A Simple Iterative C++ program to reverse
// a string
#include <bits/stdc++.h>
using namespace std;

// Function to reverse a string
void reverseStr(string& str)
{
    int n = str.length();

    // Swap character starting from two
    // corners
    for (int i=0, j=n-1; i<j; i++,j--)
        swap(str[i], str[j]);
}

// Driver program
int main()
{
    string str = "geeksforgeeks";
    reverseStr(str);
    cout << str;
    return 0;
}

Java

//A Simple Iterative Java program to reverse
//a string
class GFG {

    //Function to reverse a string
    static void reverseStr(String str)
    {
     int n = str.length();
     char []ch = str.toCharArray();
     char temp;

     // Swap character starting from two
     // corners
     for (int i=0, j=n-1; i<j; i++,j--)
     {
         temp = ch[i];
         ch[i] = ch[j];
         ch[j] = temp;
     }


     System.out.println(ch);
    }

    //Driver program
    public static void main(String[] args) {

        String str = "geeksforgeeks";
         reverseStr(str);
    }
}
// This code is contributed by Ita_c.

Python3

# A Simple Iterative Python program to
# reverse a string

# Function to reverse a string
def reverseStr(str):
    n = len(str)

    i, j = 0, n-1

    # Swap character starting from
    # two corners
    while i < j:
        str[i], str[j] = str[j], str[i]

        i += 1
        j -= 1


# Driver code
if __name__ == "__main__":
    str = "geeksforgeeks"

    # converting string to list
    # because strings do not support
    # item assignment
    str = list(str)
    reverseStr(str)

    # converting list to string
    str = ''.join(str)

    print(str)

# This code is contributed by
# sanjeev2552

C#

// A Simple Iterative C# program
// to reverse a string
using System;

class GFG
{

    //Function to reverse a string
    static void reverseStr(String str)
    {
        int n = str.Length;
        char []ch = str.ToCharArray();
        char temp;

        // Swap character starting from two
        // corners
        for (int i=0, j=n-1; i<j; i++,j--)
        {
            temp = ch[i];
            ch[i] = ch[j];
            ch[j] = temp;
        }
        Console.WriteLine(ch);
    }

    //Driver program
    public static void Main(String[] args)
    {
        String str = "geeksforgeeks";
        reverseStr(str);
    }
}

// This code is contributed by PrinciRaj1992

PHP

<?php
// A Simple Iterative PHP
// program to reverse a string

// Function to reverse a string
function reverseStr (&$str)
{
    $n = strlen($str);

    // Swap character starting
    // from two corners
    for ($i = 0, $j = $n - 1;
         $i < $j; $i++, $j--)
        //swap function
        list($str[$i],
             $str[$j]) = array($str[$j],
                               $str[$i]);
}

// Driver Code
$str = "geeksforgeeks";
reverseStr($str);
echo $str;

// This code is contributed by ajit.
?>

Javascript

<script>
//A Simple Iterative Javascript program to reverse
//a string

//Function to reverse a string
function reverseStr(str)
{
    let n = str.length;
     let ch = str.split("");
     let temp;

     // Swap character starting from two
     // corners
     for (let i=0, j=n-1; i<j; i++,j--)
     {
         temp = ch[i];
         ch[i] = ch[j];
         ch[j] = temp;
     }


     document.write(ch.join("")+"<br>");
}

//Driver program
let str = "geeksforgeeks";
reverseStr(str);

// This code is contributed by rag2127
</script>

Output:

skeegrofskeeg

Time complexity : O(n) 
Auxiliary Space : O(1)

Method 4 (Recursive)

C++

// Recursive C++ program to reverse a string
#include <bits/stdc++.h>
using namespace std;

void recursiveReverse(string &str, int i = 0)
{
    int n = str.length();
    if (i == n / 2)
        return;
    swap(str[i], str[n - i - 1]);
    recursiveReverse(str, i + 1);
}

// Driver program
int main()
{
    string str = "geeksforgeeks";
    recursiveReverse(str);
    cout << str;
    return 0;
}

Java

// Recursive Java program to reverse a string
class GFG
{

static void recursiveReverse(char[] str, int i)
{
    int n = str.length;
    if (i == n / 2)
        return;
    swap(str,i,n - i - 1);
    recursiveReverse(str, i + 1);
}
static void swap(char []arr, int i, int j)
{
    char temp= arr[i];
    arr[i]=arr[j];
    arr[j]=temp;
}

// Driver program
public static void main(String[] args)
{
    char[] str = "geeksforgeeks".toCharArray();
    recursiveReverse(str,0);
    System.out.println(String.valueOf(str));
}
}

// This code is contributed by 29AjayKumar

Python3

# Recursive Python program to reverse a string

def recursiveReverse(str, i = 0):
    n = len(str)

    if i == n // 2:
        return

    str[i], str[n-i-1] = str[n-i-1], str[i]

    recursiveReverse(str, i+1)

if __name__ == "__main__":
    str = "geeksforgeeks"

    # converting string to list
    # because strings do not support
    # item assignment
    str = list(str)
    recursiveReverse(str)

    # converting list to string
    str = ''.join(str)
    print(str)

# This code is contributed by
# sanjeev2552

C#

// Recursive C# program to reverse a string
using System;

public class GFG
{

static void recursiveReverse(char[] str, int i)
{
    int n = str.Length;
    if (i == n / 2)
        return;
    swap(str,i,n - i - 1);
    recursiveReverse(str, i + 1);
}
static char[] swap(char []arr, int i, int j)
{
    char temp= arr[i];
    arr[i]=arr[j];
    arr[j]=temp;
    return arr;
}

// Driver program
public static void Main(String[] args)
{
    char[] str = "geeksforgeeks".ToCharArray();
    recursiveReverse(str,0);
    Console.WriteLine(String.Join("",str));
}
}
// This code is contributed by Princi Singh

C++

// A quickly written program for reversing a string
// using reverse()
#include<bits/stdc++.h>
using namespace std;
int main()
{
   string str = "geeksforgeeks";

   // Reverse str[begin..end]
   reverse(str.begin(),str.end());

   cout << str;
   return 0;
}

Java

// A Simple Java program
// to reverse a string

class GFG
{

    public static void main(String[] args)
    {
        String str = "geeksforgeeks";

        // Reverse str[begin..end]
        str = reverse(str);

        System.out.println(str);
    }

    static String reverse(String input)
    {
        char[] temparray = input.toCharArray();
        int left, right = 0;
        right = temparray.length - 1;

        for (left = 0; left < right; left++, right--)
        {
            // Swap values of left and right
            char temp = temparray[left];
            temparray[left] = temparray[right];
            temparray[right] = temp;
        }
        return String.valueOf(temparray);
    }
}

// This code is contributed by 29AjayKumar

Python

# A Simple python program
# to reverse a string

# Function to
# reverse a string
def reverseStr(str):

    # print the string
    # from last
    print(str[::-1])

# Driver Code
def main():
    str = "geeksforgeeks";
    reverseStr(str);

if __name__=="__main__":
    main()

# This code is contributed
# by prabhat kumar singh

C#

// A Simple C# program to reverse a string
using System;

class GFG
{

    public static void Main(String[] args)
    {
        String str = "geeksforgeeks";

        // Reverse str[begin..end]
        str = reverse(str);

        Console.WriteLine(str);
    }

    static String reverse(String input)
    {
        char[] temparray = input.ToCharArray();
        int left, right = 0;
        right = temparray.Length - 1;

        for (left = 0; left < right; left++, right--)
        {
            // Swap values of left and right
            char temp = temparray[left];
            temparray[left] = temparray[right];
            temparray[right] = temp;
        }
        return String.Join("",temparray);
    }
}

/* This code is contributed by PrinciRaj1992 */

PHP

<?php
// A Simple PHP program
// to reverse a string

// Function to reverse a string
function reverseStr($str)
{

    // print the string
    // from last
    echo strrev($str);
}

// Driver Code
$str = "geeksforgeeks";

reverseStr($str);

// This code is contributed
// by Srathore
?>

Output:

skeegrofskeeg

Time complexity: O(n) where n is length of string

Auxiliary Space: O(n)

Method 5: Using the inbuilt method ‘.reverse()’ of StringBuffer class in Java

The idea is to initialise an object of the StringBuffer class using the given string which needs to be reversed and then invoke the .reverse() method. After doing so, you can convert the StringBuffer to a string by using the toString() function.

Below is the implementation of the above idea:

Java

//Java program to reverse a string using StringBuffer class

import java.io.*;
import java.util.*;

class GFG {

    //Driver Code
    public static void main (String[] args) {
        String str = "geeksforgeeks";//Input String

        //Step 1: Initialise an object of StringBuffer class
        StringBuffer sb = new StringBuffer(str);

        //Step 2: Invoke the .reverse() method
        sb.reverse();

        //Step 3: Convert the StringBuffer to string by using toString() method
        System.out.println(sb.toString());

    }
}

//This code is contributed by shruti456rawal

Output

skeegrofskeeg

Time Complexity: O(n)