Reverse a string preserving space positions

Last Updated : 06 Mar, 2025

Given a string s, the task is to reverse the given string while preserving the position of spaces.

Examples:

Input: "internship at geeks for geeks"
Output: skeegrofsk ee gtapi hsn retni
Explanation : Reversing the characters without spaces "skeegrofskeegtapihsnretni" and inserting space at original place"skeegrofsk ee gtapi hsn retni"
Input : "abc de"
Output: edc ba
Explanation : Reversing the characters without spaces "edcba" and inserting space at original place"edc ba"
Input "Help others"
Output: sreh topleH
Explanation : Reversing the characters without spaces "srehtopleH" and inserting space at original place"sreh topleH"

[Naive Approach] - Using a new String - O(n) Time and O(n) Space

The idea is to create a string to store results. Mark the space position of the given string in this string and then start Inserting the character from the input string into the result string in reverse order.
While inserting the character check if the result string already has a space at index ‘j’ or not. If it does, we copy the character to the next position.

C++

#include <iostream>
using namespace std;

string reverses(string &str)
{
    // Mark spaces in result
    int n = str.size();
    string result(n, '\0');
    for (int i = 0; i < n; i++)
        if (str[i] == ' ')
            result[i] = ' ';

    // Traverse input string from beginning
    // and put characters in result from end
    int j = n - 1;
    for (int i = 0; i < str.length(); i++) {
        
        // Ignore spaces in input string
        if (str[i] != ' ') {
            
            // ignore spaces in result.
            while(result[j] == ' ')
                j--;

            result[j] = str[i];
            j--;
        }
    }
    return result;
}

int main()
{
    string s = "internship at geeks for geeks";
    cout << reverses(s) << endl;
    return 0;
}

Java

public class GfG{
   
    static void reverses(String str)
    {

        char[] inputArray = str.toCharArray();
        char[] result = new char[inputArray.length];

        // Mark spaces in result
        for (int i = 0; i < inputArray.length; i++) {
            if (inputArray[i] == ' ') {
                result[i] = ' ';
            }
        }

        // Traverse input string from beginning
        // and put characters in result from end
        int j = result.length - 1;
        for (int i = 0; i < inputArray.length; i++) {

            // Ignore spaces in input string
            if (inputArray[i] != ' ') {

                // ignore spaces in result.
                if (result[j] == ' ') {
                    j--;
                }
                result[j] = inputArray[i];
                j--;
            }
        }
        System.out.println(String.valueOf(result));
    }

    public static void main(String[] args)
    {
        String s = "internship at geeks for geeks";
        reverses(s);
    }
}

Python

def reverses(st):

    # Mark spaces in result
    n = len(st)
    result = [0] * n
    
    for i in range(n):
        if (st[i] == ' '):
            result[i] = ' '

    # Traverse input string from beginning
    # and put characters in result from end
    j = n - 1
    for i in range(len(st)):
        
        # Ignore spaces in input string
        if (st[i] != ' '):
            
            # Ignore spaces in result.
            if (result[j] == ' '):
                j -= 1

            result[j] = st[i]
            j -= 1
            
    return ''.join(result)

if __name__ == "__main__":

    s = "internship at geeks for geeks"
    print(reverses(s))

using System;

class GfG {

    // Function to reverse the string
    // and preserve the space position
    static void reverses(string str)
    {
        char[] inputArray = str.ToCharArray();
        char[] result = new char[inputArray.Length];

        // Mark spaces in result
        for (int i = 0; i < inputArray.Length; i++) {
            if (inputArray[i] == ' ') {
                result[i] = ' ';
            }
        }

        // Traverse input string from beginning
        // and put characters in result from end
        int j = result.Length - 1;
        for (int i = 0; i < inputArray.Length; i++) {

            // Ignore spaces in input string
            if (inputArray[i] != ' ') {

                // ignore spaces in result.
                if (result[j] == ' ') {
                    j--;
                }
                result[j] = inputArray[i];
                j--;
            }
        }
        for (int i = 0; i < result.Length; i++)
            Console.Write(result[i]);
    }

    public static void Main()
    {
        string s = "internship at geeks for geeks";
        reverses(s);
    }
}

JavaScript

function reverses(str)
{
    let inputArray = str.split("");
        let result = new Array(inputArray.length);
 
        // Mark spaces in result
        for (let i = 0; i < inputArray.length; i++) {
            if (inputArray[i] == ' ') {
                result[i] = ' ';
            }
        }
 
        // Traverse input string from beginning
        // and put characters in result from end
        let j = result.length - 1;
        for (let i = 0; i < inputArray.length; i++) {
 
            // Ignore spaces in input string
            if (inputArray[i] != ' ') {
 
                // ignore spaces in result.
                if (result[j] == ' ') {
                    j--;
                }
                result[j] = inputArray[i];
                j--;
            }
        }
        return (result).join("");
}

let s = "internship at geeks for geeks";
console.log(reverses(s));

Output

skeegrofsk ee gtapi hsn retni

[Expected Approach] - Using two Pointers - O(n) Time and O(1) Space

The idea is to use two pointers pointing at start and end of the string. If character at start or end is space, we move the pointer pointing to the space to the next position and swap only if both pointer point to a character.

C++

#include <iostream>
using namespace std;

string preserveSpace(string &str)
{
    int n = str.length();

    // Initialize two pointers as two corners
    int start = 0;
    int end = n - 1;

    // Move both pointers toward each other
    while (start < end) {

        // If character at start or end is space,
        // ignore it
        if (str[start] == ' ') {
            start++;
            continue;
        }
        else if (str[end] == ' ') {
            end--;
            continue;
        }

        // If both are not spaces, do swap
        else {
            swap(str[start], str[end]);
            start++;
            end--;
        }
    }
    return str;
}

int main()
{
    string s = "internship at geeks for geeks";
    cout << preserveSpace(s);
    return 0;
}

Java

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

class GFG{
    
public static String preserveSpace(String str)
{
    int n = str.length();
 
    // Initialize two pointers as two corners
    int start = 0;
    int end = n - 1;
    
    char[] Str = str.toCharArray();
    
    // Move both pointers toward each other
    while (start < end)
    {
        
        // If character at start or end 
        // is space, ignore it
        if (Str[start] == ' ') 
        {
            start++;
            continue;
        }
        else if (Str[end] == ' ') 
        {
            end--;
            continue;
        }
 
        // If both are not spaces, do swap
        else 
        {
            char temp = Str[start];
            Str[start] = Str[end];
            Str[end] = temp;
            start++;
            end--;
        }
    }
    return String.valueOf(Str);
}

public static void main(String[] args) 
{
    String s = "internship at geeks for geeks";
    System.out.println(preserveSpace(s));
}
}

Python

def preserveSpace(Str):
  
    n = len(Str)
    Str = list(Str)

    # Initialize two pointers 
    # as two corners 
    start = 0
    end = n - 1

    # Move both pointers 
    # toward each other 
    while(start < end):

        # If character at start 
        # or end is space, 
        # ignore it 
        if(Str[start] == ' '):
            start += 1
            continue
        elif(Str[end] == ' '):
            end -= 1
            continue
            
        # If both are not 
        # spaces, do swap 
        else:
            Str[start], Str[end] = (Str[end],
                                    Str[start])
            start += 1
            end -= 1 
    return (''.join(Str))

s = "internship at geeks for geeks"
print(preserveSpace(s));

using System;
using System.Collections.Generic; 

class GfG{
    
static string preserveSpace(string str)
{
    int n = str.Length;
    
    // Initialize two pointers 
    // as two corners
    int start = 0;
    int end = n - 1;
     
    char[] Str = str.ToCharArray();
     
    // Move both pointers toward 
    // each other
    while (start < end)
    {
        
        // If character at start or 
        // end is space, ignore it
        if (Str[start] == ' ') 
        {
            start++;
            continue;
        }
        else if (Str[end] == ' ') 
        {
            end--;
            continue;
        }
  
        // If both are not spaces, do swap
        else
        {
            char temp = Str[start];
            Str[start] = Str[end];
            Str[end] = temp;
            start++;
            end--;
        }
    }
    return new string(Str);
}
  
static void Main()
{
    string s = "internship at geeks for geeks";
 
    Console.Write(preserveSpace(s));
}
}

JavaScript

function preserveSpace(str)
    {
        let n = str.length;

        // Initialize two pointers
        // as two corners
        let start = 0;
        let end = n - 1;

        let Str = str.split('');

        // Move both pointers toward
        // each other
        while (start < end)
        {

            // If character at start or
            // end is space, ignore it
            if (Str[start] == ' ')
            {
                start++;
                continue;
            }
            else if (Str[end] == ' ')
            {
                end--;
                continue;
            }

            // If both are not spaces, do swap
            else
            {
                let temp = Str[start];
                Str[start] = Str[end];
                Str[end] = temp;
                start++;
                end--;
            }
        }
        return Str.join("");
    }
    
    let s = "internship at geeks for geeks";
    console.log(preserveSpace(s));

Output

skeegrofsk ee gtapi hsn retni

Remove duplicates from a string in O(1) extra space

Rishabh Jain

Improve

Article Tags :

Practice Tags :

Reverse a string preserving space positions

[Naive Approach] - Using a new String - O(n) Time and O(n) Space

[Expected Approach] - Using two Pointers - O(n) Time and O(1) Space

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