Reverse a string preserving space positions

Write a program to reverse the given string while preserving the position of spaces.

Examples: 

Input  : "abc de"
Output : edc ba

Input : "intern at geeks"
Output : skeegt an retni

Input : "Help others"
Output : sreh topleH

1. Create a string to store results. Mark the space position of the given string in this string.
2. Insert the character from the input string into the result string in reverse order.
3. While inserting the character check if the result string already contains a space at index ‘j’ or not. If it contains, we copy the character to the next position.

Below is the implementation of the above steps. 

C++

// C++ program to reverse a string preserving
// spaces.
#include <iostream>
using namespace std;

// Function to reverse the string
// and preserve the space position
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;
}

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

Java

// Java program to reverse a string
// preserving spaces.
public class ReverseStringPreserveSpace {
    // 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--;
            }
        }
        System.out.println(String.valueOf(result));
    }

    // driver function
    public static void main(String[] args)
    {
        reverses("internship at geeks for geeks");
    }
}

// This code is contributed by Rishabh Jain

Python3

# Python3 program to reverse a string preserving
# spaces.

# Function to reverse the string
# and preserve the space position
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)

# Driver code
if __name__ == "__main__":

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

# This code is contributed by ukasp

C#

// C# program to reverse a
// string preserving spaces.
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]);
    }

    // Driver code
    public static void Main()
    {
        reverses("internship at geeks for geeks");
    }
}

// This code is contributed by Sam007

Javascript

<script>
// Javascript program to reverse a string
// preserving spaces.

// Function to reverse the string
    // and preserve the space position
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--;
            }
        }
        document.write((result).join(""));
}
// driver function
reverses("internship at geeks for geeks");


// This code is contributed by ab2127
</script>

Output:

skeegrofsk ee gtapi hsn retni

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

Optimized Solution:

The idea is to use two pointers to reverse.  

C++

// C++ program to implement
// the above approach
#include <iostream>
using namespace std;

void 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--;
        }
    }
}

// Driver code
int main()
{
    string str = "internship at geeks for geeks";
    preserveSpace(str);
    cout << str;
    return 0;
}

Java

// Java program to implement
// the above approach
import java.io.*;
import java.util.*;

class GFG{

public static void 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--;
        }
    }
    System.out.println(String.valueOf(Str));
}

// Driver Code
public static void main(String[] args)
{
    String str = "internship at geeks for geeks";

    preserveSpace(str);
}
}

// This code is contributed by divyeshrabadiya07

Python3

# Python3 program to implement
# the above approach
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
    print(''.join(Str))

# Driver code
Str = "internship at geeks for geeks"
preserveSpace(Str);

# This code is contributed by avanitrachhadiya2155

C#

// C# program to implement
// the above approach
using System;
using System.Collections.Generic;

class GFG{

static void 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--;
        }
    }
    Console.Write(new string(Str));
}

// Driver code
static void Main()
{
    string str = "internship at geeks for geeks";

    preserveSpace(str);
}
}

// This code is contributed by divyesh072019

Javascript

<script>

    // Javascript program to implement
    // the above approach

    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--;
            }
        }
        document.write(Str.join(""));
    }

    let str = "internship at geeks for geeks";

    preserveSpace(str);

</script>

Output:

skeegrofsk ee gtapi hsn retni

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

This article is contributed by Rishabh Jain. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.