Reverse a String

Given a string, write a recursive program to reverse it

String Reverse

Reverse String using Stack:

The idea is to push all the characters of the string in a stack. Then pop the elements of the stack and store it in the initial string from the starting index. Since the stack is based on last in first out, so the string formed will be reverse of the original string.

Below is the implementation of the above approach:

C++

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

void reversebyStack(string &str)
{
  // To store the characters of original string.
   stack<char> st;
   for (int i=0; i<str.length(); i++)
     // Push the charcters into stack
       st.push(str[i]);

   for (int i=0; i<str.length(); i++) {
     // Pop the charcters of stack into the original string.
       str[i] = st.top();
       st.pop();
   }
}

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

Java

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

   for (int i=0; i<str.length; i++) {
      // Pop the charcters of stack into the original string.
    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 = reversebyStack(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 reversebyStack(str):

    # using as stack
    stack = []

    for i in range(len(str)):
      # Push the charcters into stack
        stack.append(str[i])

    for i in range(len(str)):
       # Pop the charcters of stack into the original string.
        str[i] = stack.pop()

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

    # converting string to list
    # because strings do not support
    # item assignment
    str = list(str)
    reversebyStack(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 reversebyStack(char []str)
    {
        Stack<char> st = new Stack<char>();
        for(int i = 0; i < str.Length; i++)
          // Push the charcters into stack
            st.Push(str[i]);

        for (int i = 0; i < str.Length; i++)
        {
           // Pop the charcters of stack into the original string.
            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 = reversebyStack(str.ToCharArray());
        Console.WriteLine(str);
    }
}

// This code is contributed by Rajput-Ji

Javascript

// Function to reverse a string using a stack
function reverseByStack(str) {
    // To store the characters of the original string.
    const stack = [];

    // Push the characters into the stack.
    for (let i = 0; i < str.length; i++) {
        stack.push(str[i]);
    }

    // Pop the characters of the stack and build the reversed string.
    let reversedStr = '';
    while (stack.length > 0) {
        reversedStr += stack.pop();
    }

    return reversedStr;
}

// Driver program
function main() {
    // Original String
    let str = "geeksforgeeks";
    let reversedStr = reverseByStack(str);
    console.log(reversedStr);
}

main();


// This code is contributed by shivamgupta0987654321

Output

skeegrofskeeg

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

Reverse String using two pointers:

The idea is to use two pointers. The left pointer is placed at the beginning of the string and the right pointer is placed at the end of the string. Now swap the characters at left and right pointers, after that left pointer moves forward by 1 step and right pointer moves backward by 1 step. This operation is continued till right pointer is ahead of left pointer.

Below is the implementation of the above approach:

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
    // i is the left pointer and j is the right pointer
    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
import java.io.*;
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
     // i is the left pointer and j is the right pointer
     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
    # i is the left pointer and j is the right pointer
    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
        // i is the left pointer and j is the right pointer
        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

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
     // i is the left pointer and j is the right pointer
     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>

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.
?>

Output

skeegrofskeeg

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

Reverse String using Recursion:

The recursive algorithm to reverse a string works by swapping the first and last characters until the middle of the string is reached. This process is performed through recursive calls, where in each call, characters at positions i and n-i-1 are swapped, and i is incremented. This continues until i reaches n/2, and the string is completely reversed.

Below is the implementation of the above approach:

C++

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

// Recursive function to reverse the string
void recursiveReverse(string &str, int i = 0)
{
    int n = str.length();
    if (i == n / 2)
        return;
  // Swap the i and n-i-1 character
    swap(str[i], str[n - i - 1]);
  // Call Recursive function after incrementing i.
    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
import java.io.*;
class GFG
{

  // Recursive function to reverse the string
static void recursiveReverse(char[] str, int i)
{
    int n = str.length;

    if (i == n / 2)
        return;

  // Swap the i and n-i-1 character
    swap(str,i,n - i - 1);

  // Call Recursive function after incrementing i.
    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
    # Swap the i and n-i-1 character
    str[i], str[n-i-1] = str[n-i-1], str[i]

    # Call Recursive function after incrementing 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 the i and n-i-1 character
    swap(str,i,n - i - 1);

  // Call Recursive function after incrementing i.
    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

Javascript

// Recursive JavaScript function to reverse a string
function recursiveReverse(str, i = 0) {
    const n = str.length;
    if (i >= Math.floor(n / 2))
        return str;
    // Swap the i and n-i-1 characters
    str = str.substring(0, i) + str[n - i - 1] + str.substring(i + 1, n - i - 1) + str[i] + str.substring(n - i);
    // Call Recursive function after incrementing i.
    return recursiveReverse(str, i + 1);
}

// Driver program
    let str = "geeksforgeeks";
    str = recursiveReverse(str);
    console.log(str);

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)

Reverse String using inbuilt method

Below is the implementation to reverse a string using built-in methods in different programming languages:

C++

#include <bits/stdc++.h>
using namespace std;

int main()
{
    string str = "geeksforgeeks"; // Input string
    reverse(str.begin(),str.end()); // Reverse the string
    cout << str << std::endl;
    return 0;
}


// This code is contributed by prajwalkandekar123.

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

Python3

# Driver Code
if __name__ == '__main__':
    str = "geeksforgeeks" # Input String

    # Step 1: Initialise an object of StringBuffer class
    sb = str[::-1]

    # Step 2: Invoke the .reverse() method (not applicable in Python)

    # Step 3: Print the reversed string
    print(sb)

C#

// c# code

using System;

class Program {
    static void Main(string[] args)
    {
        string str = "geeksforgeeks"; // Input string
        char[] charArray
            = str.ToCharArray(); // Convert string to char
                                 // array
        Array.Reverse(charArray); // Reverse the array
        str = new string(
            charArray); // Convert char array back to string
        Console.WriteLine(
            str); // Output the reversed string
    }
}
// ksam24000

Javascript

let str = "geeksforgeeks"; // Input string
str = str.split('').reverse().join(''); // Reverse the string
console.log(str);

// This code is contributed by Prajwal Kandekar

Output

skeegrofskeeg

Time Complexity: O(n)
Auxiliary Space: O(1) in all the programming languages except Java in which it will be, O(n) (The extra space is used to store the StringBuffer string).